What a timely question. Here's my story.
Just bought a M45 Campie and had to completely disassemble it after the mag disconnect froze up the system. Not too hard, but you really need to be ready for some cussing. Make sure you have enough slave pins, too. Knowing the system, now, I wouldn't hesitate to disassemble mine again.
Anyhow, you definitely need to go with the 16.5# recoil spring. Why they use that wimpy 11# spring is a question I can't find the answer to, especially when 9mm/45acp pistols don't use recoil springs that light.
Wolff has both heavy-weight springs in stock, and they are dirt cheap.
www.gunsprings.com
As for the buffer, a guy on one of these sights, probably warriortalk forum, talked about whittling one out of the tread from a semi's tire. He says it works great, better than the original. Sounds like a bit of work for me, but a call to Marlin showed that you can only get one new buffer, as they are running out. And, get this, they want you to send the old one back to them so they can see what's going wrong with them.
I would venture to say that cleaning agents, light recoil spring, powder residue and age all contribute to the demise of the polymer buffer.
Any other parts you'll want to stock up on? You definitely want to get a Hammer Strut and a Hammer Strut Bridge. Unless you have machining capabilities and want to make better, thicker ones on your own. The HS is very thin and mine has already developed a bow, even though the recoil spring surrounds and supports it. The HSB is very weak, being too thin and the only thing supporting the other end of the recoil spring (it bent noticeably after only about 100 rounds). I'm looking to laminate the two that I have with some heavy epoxy.
All parts are available directly from Marlin and don't cost an arm and leg.
Other than that, I can't think of anything that is easy to fix. The magazine release button is teeny and hard to work, definitely not something for speed reloads, but what can be done? The mag-disconnect safety is good if the parts stay in line, and it's not too hard to get rid of it all together. Accuracy is great, though I'd like better iron sights for these old eyes. And if you run into some type of barrel shroud that will work, something similar to that on the .30cal M1 carbine, do holler my way, please.