My wife was given a 3GS to use with her Straight Talk account a few months ago. It had a shattered digitizer (front glass) and a missing volume button, not to mention a few spots of separation around the chrome trim. The digitizer replacement is pretty easy, as long as you're patient and careful. The volume key is much harder, as it involves removing almost every component in the phone to get to the switch itself.
To simplify things a LOT, I simply glued the chrome "button" to the still-working switch. If yours still works (I could press my wife's with a paperclip and change the volume) then this may be the easiest, fastest, and cheapest solution.
Be glad its not an iPhone 4 (repair-wise, at least) - replacing THAT front glass is a nightmare! Recently did that for my bro-in-law and I had to completely disassemble his 4; good news is all is good now.
Last, as for the back plastic, are you going to use a case? If so, it may be best to not even worry about the back; after all, it will be hidden. That is, if its structurally sound... I'm starting my second 3GS repair this week (after the parts come in); I received another busted-but-working one for myself (to replace my 3G and tide me over until the new iPhone is released) and even though the back does have some cracks, I'm not going to worry about it.
BTW - I ordered all my parts from eBay, for both the 3GS and the 4. Bad news was that BOTH of them had to have replacement parts shipped out; the 3GS digitizer had about 1/2" at the bottom that wouldn't respond to touch and the 4 glass cracked a few minutes after install without even being touched (it was lying on a table). Good news, though, as that both sellers replaced without question; I was worried about the 4 because of the cracks in it; it was fine after the install, but had cracked an hour later... Best of luck!
