I getting a 9mm upper that's complete with 9mm bolt with mag well adapter and I want to put it on a 5.56 colt lower.
1. Can I use my existing lower the way it's set-up (5.56) or will it require a different hammer, springs, etc..?
2. 9mm Buffer - I'm guessing I'll need one with a 9mm recoil spring too?
Depends: If you got the Colt-style system, you'll need to replace your hammer with one that doesn't have the little "step" on the face of it. It can STILL BE a .223 hammer, and you don't have to get the 9mm one. If your existing .223 hammer has a completely flat face, don't worry about it. It'll work just fine. As far as the buffer and spring, just stick with the regular ones. They work just fine.