I replaced a cracked screen on my old iPhone 3gs a while ago. That was immediately followed by replacing the digitiser which I broke in the process.
Getting into the phone is a bit daunting as you have to crack it open, but after that it's not too bad as everything just plugs in.
On balance though I reckon it's worth leaving someone else to deal with the learning curve and next time I took it to a local repairer.
If the phone is in warranty then an Apple Store will probably just swap it out for you, which has the benefit of getting you a new battery. If you've had the phone for a while then it might be worth getting a new battery fitted while you've got it in pieces.