Thanks! In terms of MP G1 Optimus Primes, you have several options.
Official:
MP-01 Optimus: the first Masterpiece figure, it was revolutionary for its time but honestly hasn't aged well, and doesn't scale with current MPs.
MP-10 Optimus: A very solid figure but since the MP line has shifted to a cartoon style, it might look out of place with the newest MPs.
MP-44 Optimus: The current MP Optimus. Looks solid but in my opinion it's overengineered, leading to breakages in the knees. Additionally, while it's coated in glossy paint, it also makes it more prone to paint chipping. It's also extremely expensive as you've said.
3rd party:
Transform Element TE-01: This is the one I posted. It's heavy and chunky due to die-cast components. It's very cartoon accurate and features a gun with an LED.
Magic Square MS-01: It's been discontinued for a while so prices on the aftermarket have gone up like crazy. Not as heavy as the TE-01 and not as accurate, although it is highly articulated.
There are also KO versions of MP-10 and MP-44, but the MP-44 KO is made of very soft plastic, which has caused a lot of breakages.
My recommendation is the TE-01. It'll set you back $105-$150 depending on the retailer, but it comes with everything you need: 2 heads depicting different cartoon eras, blaster, and axe. You should still be able to find it on ebay. Hope this helps!