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?

Thanks,
PBR

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.