The cheapest way of acquiring a full frame sensor is still via a film camera. I just recently purchased two ultra compact Nikon FGg's for everyday shooting that were in practically mint cosmetic and mechanical condition for a combined purchase price of $150. Nevertheless, if you're going to purchase any vintage camera (anything over twenty years old for purpose of argument), they will more likely than not need to have the light seals resealed- despite the description on Ebay stating that they're just fine. Closer inspection will often reveal a crumbling residue transitioning into one gooey mess.
Now, you can either send it out to be professionally resealed ($50 to $100), or choose to do it yourself. That's where Jon Goodman comes in- he supplies handy, pro quality DIY resealing kits custom cut for specific makes and models at extremely reasonable prices via Ebay complete with instructions, illustrations and scraping tool (you'll also need some lighter fluid).
Word of Caution: This is not a task for those of you with ham hocks for paws. Even those with delicate digits like mine will find this a chore requiring considerable patience, concentration and care. Cleaning the old seals releases sticky crumbs that can attach to the shutter curtain- get the picture? So take adequate precautions. Once done cleaning, installing the new seals is relatively easy since they fit readily into place.
Unless you're fortunate enough to have a camera with removable, interchangeable focusing screens, replacing the mirror dampener is where it really hits the fan. Since you don't want falling crumbs from the old dampener falling onto the focusing screen, you really want it outta there. So seriously consider having a professional do this part- and since you've already replaced the light seals, it won't cost as much.
If you're hell bent on doing it yourself however, as was I, you'll definitely need a 00 Phillips head and tweezers to remove the focusing screen from the bracket. Getting it out is fairly easy, repositioning and replacing it in the bracket properly is considerably more difficult. One of my FG's wouldn't cooperate and the screen suffered a few minor scratches along the edges, the second went much more smoothly without any damage- I'd like to think it was experience, but I know it was just pure luck. And even a cheapo vintage SLR viewfinder runs circles around those of $1,500 DSLR's.
The following video highlights the procedure (although it also makes it appear as if it's something that can be done while watching Monday Night Football)...