Confused as to which iPhones require what parts.

There are several generations of the IPhone with different models for that generation i.e. Verizon vs AT&t. I'm trying to start a local IPhone repair business at my college specifically dealing with cracked screens.

As I understand you can buy separate the lcd, front panel, or whole display assembly for the IPhone up to the 3gs model and you can successfully transfer the home button and speaker from the would be broken display to the knew display, reattach, and then the phone should work. So you are essentially buying what needs to be repaired, transferring what still works on the old display to the new iphone and then reassembling. However you need to buy separate types of front panels for the 3g and 3gs as they aren't compatible.

However for the newer Iphone 4, 4s models, you have to buy the digitizer + LCD assembly as you can't buy any of these things separate. Meaning you can't dismantle the home button, speaker, or front panel from the broken Iphone and transfer as you would like the 3g/s models?

Which then asks the question, do you have to buy specific parts for the IPhone 4 AT&T version? I'm wondering which parts are compatible with what versions. Assuming I have to buy the digitizer + LCD assembly, do I need to buy a separate type for each IPhone 4, 4s, Verizon?

I'm a little confused, or maybe I'm right on the money. If someone would educate me as to what I'm concluding correctly or incorrectly I would be much obliged. Gracias.