What Are The Best "Project" AEG's To Look For (so I can learn tech without molesting my primary)

Another user posted today about getting into tech. It made me think about how much I would love to work on my gun, but I already learned the hard way with my hotrodding habit...never tinker with your daily driver (or in this case shooter).

So I'm going to leave my primary alone, but I still need something to learn on. I have seen the advice about picking up boneyard guns, I'm not against this but there are some pros and cons here as the damage could be irreparable and then there is some wasted time and money. I know if I go boneyard I need a Gen II or III gearbox. However I don't know which guns have which, and I would be at the mercy of google or the seller to find out.

What are some good, fairly universal, AEG models I should be looking for in order to learn everything about AEG's. I am a hands on learner and I have read quite a bit but I need to lay hands on something to learn it. I'm looking for decent budget offerings that would give me the chance to learn while I upgrade/fix them.

Secondary question: I have all the tools needed to work on cars, are there any special airsoft tools I need to get (things I won't already have in the box).