I'm starting up with iPhone repair and I'm looking for advice on parts to stock.

I get that LCD/Digitizers, battery's, and dock connectors are the most common. I'm wondering if I should stock many parts other than those.

The problem is that I live in a very remote spot that would require waiting weeks for parts if I order them on a case by case basis. So I would like to be able to help my customers as quickly as possible, while also not ordering a bunch of parts that are really unnecessary. Is it better to have multiple working phones to use for testing or is it pretty rare to receive a phone that is need of a new part or flex cable? Does doing work on water damaged phone's change any of that?

I've done plenty of teardowns and LCD/Digitzer replacement, but haven't had any iPhone troubleshooting cases and I'm not sure if it is as regular an issue as with a Mac. Any help is appreciated. Thanks in advance.