Yes, Asgardianboy. As Manannin said, you asked for a TON of info. It is dangerously close to asking someone to write your research paper for you.
That said, off the top of my head...
1 - *see bottom of this post for my notes/additions/tweaks to first page info*
2 - I wasn't around in the early days, but I know of no other thread here or the OSW that would have a more condensed version of this info.
3.1 - Regarding any bodies, try OSW for the most current info.
Seamless male bodies have not come very far last I checked They always lack mobility and longevity. I haven't tried them all, but I can tell you the "D-Boy" Mogadishu body is not a good choice. The figure itself is too short (can't have Hicks be the shortest guy in the room!) and the wires pop out the fingers.
3.2 - Phycan stainless steel frame bodies seem to be the most advanced female seamless bodies. It may be a good choice for Ripley. They are supposedly nice, but I dont have any and I am also pretty sure, like most female bodies, they are pretty short and small. Weaver is 6ft, so even they wont be very accurate.
NO ONE makes a good 'buff' female body, let alone a seamless one. For my Vaz I used an old CyGirl body. They were famous for having manly looking women, so you can find some that have general 'mass' to them, but none are great. In short, keep dreaming. 1/6th companies have their hands full with sexy/busty skinny chicks. It is unlikely anyone ever makes a 'buff' female.
3.3 - Take a stroll down the doll isle. Barbie is getting some competition lately and some of it is a bit more realistic. I think modifying one of those is your best bet. High end replica 7 year old children is a weird thing man. I mean "WE" know it is for newt, and not something weird. Still...I suggest you modify/build your own rather than go searching for it. Can't imagine anything good will come from having that in your browser history!
3.4 - Snyderman's sculpts on ebay are pricey, but likely the best out there. Bill Kern has a nice Hudson that is also available and not expensive at all. Usually it comes unpainted though. For the most part other Hicks and Hudson's out there are either not great or simply no longer available. Some pop up on the secondary market if you keep your eyes open. The "Stoner" head and stock Hudson head can be repainted into a decent Michael Biehn. The Apone head actually is better than you might think after a repaint. I was shocked how good some people can make it look after ditching the stupid goatee and painting it properly.
3.5 - ACE, Soldier Story, and Toy soldier, PlayToy, DAM and several others all offer this type of boot now. They are not rare and go for $20-30. In a perfect world get the 'Vabrim' soled ones (not 'panama' soles).
4. - No. They have teased an updated Ripley for 2 years now and it still isn't up for pre-order (meaning another year at least). With how full the roster from the Marvel Movies and Star Wars are, it is safe to assume Hot Toys has their hands full and the marines are never coming back.
Back to your 1st question. Here are the basics. (In addition to Wor-Gar's list, and I have probably repeated a lot of what he said. Also I'm sure I forgot stuff. I will update if I remember more)
For everyone (all the Hot Toys marines)
-Ditch the grenades. Only Hicks had grenades on his right strap. None of the other Hot Toys figures they made actually wore grenades. (Only Ski and Frost even had grenades besides Hicks.)
-Cut the decorative webbing OFF the pouches. they were solid OD green canvas. Not multicolored with decorative webbing. You may need to glue back together...
-Ditch the pouches and start all over. No one had 3 pouches. Why hot toys give everyone 3 pouches I have no idea... (details in each character below)
-"Weather" the BDUs. Fade them mostly, but either way try to make them less "yellow"
-Replace the stock boots with Vabrim pattern boots (Note: Vasquez had black 'ranger' boots, not the jungle boots the men wore)
If you want to get really obsessive:
-Repaint the armor or simply paint over the 'bad colors'. No "olive drab" or that aweful yellowish green was used. Find a nice sand/tan and go over the greenish yellow and go over the olive drab with a very very dark green. Or just start all over. All good, just get those 2 colors gone.
For Apone:
-He has a boot knife (shorter than Hudsons) on his right strap, UNDER the VP70 holster.
-VP70 holster should run diagonal so the bottom ties off to the base of his left strap (not vertical)
-Flamethrower is not needed unless you are doing the scene where he roasts the hugger. He took Frost's flame thrower, so it isn't really "his" (Fine if you love that scene, just saying...)
-Ditch the 3rd "pouch" on his belt. He only had 2 (1 big, 1 small).
-Find 3 'belt tools'. One high on the left strap, one on the right hip and one on the left side.
For Hicks
-Replace Hands (or use natural hands when getting new body). He didnt wear gloves.
-Drill out the 'hole' where "Born again" is painted on his left shoulder bell
-Write "Non Toxic" over his codpiece on his ab-plate in gray
-Write his solcial security number across his upper arm (his tattoo)
-Paint "LIFER" in white between the lower brackets on his back
-Paint the red 'kanji' above "LIFER"
-Get a tan helmet band
-Attach a chin strap and tuck the bits under his helmet band (so they are pulled tight out of the way)
-Replace the cartoon style 870 shotgun he came with. (Ideally use a modded full size 21C toys Ithaca 37, but a 'stakeout' Ithaca 37 works in a pinch)
-Hicks has only 1 pouch! Ditch the 2 small ones. He only had the one big one.
-Find 1 'belt tool' for his right pelvis area.
-Run 3 grenades in right strap if going for the middle/later movie effect. 4 grenades for a "pre-crap-hits-the-fan" look.
For Hudson
-All the stuff Wor-Gar already said about his personalizations, grafitti fixes, and paracord)
-Replace hands (or leave natural hands on new body). He didn't wear gloves.
-Add a "Hack Pouch" on his right hip
-Add an "Eagle" bolo tie to his right strap
-Only 2 pouches (ditch the 2nd 'small' pouch)
-Ditch the VP70 entirely.
-Find a 'belt tool' for his left side on the belt.
-Get a Gerber style fighting knife and get a black leather sheath on that left strap
-Wrap left elbow in 'bandage' if going for a post-hive look (if you have a whining headsculpt, this is a must)
-Burn/drill some holes in the back of the left shoulder bells and flaps from when he got burned in the APC escaping from the hive (if you want a battle damaged post-hive look only, of course)
For Vasquez
-Ditch the VP70 and holster completely. Get a S&W Model 39.
-Use her OD belt and find another to build a holster chest rig so the model 39 sits under her breastplate
-Ditch ALL pouches. She has zero.
-I recommend trimming the sleeves on her BDU even further back than they are
-You can change her boots if you want, but accurate french ranger boots are not made to my knowledge. Her boots WERE black and were NOT the jungle boots the men wore, so don't stress it too much. Her stock black boots are not as terrible as they could be.
-Take the mislabeled 'batteries' she came with. They are the 'belt tools'. Hot Toys screwed that up. Give them to Hicks and Hudson!!! (1 each).
-Add a short "boot knife" tied to her right boot
For Drake
-Ditch the VP70 and holster completely.
-Ditch ALL pouches and the big olive drab belt. He has zero.
-Take the mislabeled 'batteries' he came with. They are the 'belt tools'. Give them to Apone! (Apone needs 3, but hey, its a start)
-Add a short "boot knife" tied to his LEFT boot.
-His gun should say "MY *****" on the side...(not Tom. Obviously)
-"GRRRRrrrr" shoudl be written on the right side of the trigger guard and scrolling up into the main block holding the trigger guard
-A bloodshot 'Eye ball' should be painted on to his headset eye-piece
-Some kind of symbol should be painted on to the back of his gloves
-His necklace can be wrapped around the barrel of the smartgun
