I don't have much personal experience other than watching my friends laptopping from underneath a car. www.dorikaze.com is a forum for drift racing that a lot of my friends frequent, and there's bound to be at least a couple knowledgeable people in there. Be warned though, there's a lot of grade-school antics, meming, and sexism, and the smart guys are used to curtly fending off stupid questions. So do a lot of reading before you start typing.

The best way to start is to not buy a vehicle with a hundred proprietary computers in it.

How do you search for and verify one of those vehicles without a computer?

Also, I haven't bought a new car in many years. Is there something like a EULA buried in the small print? And I don't imagine buyers on the secondary market sign anything. Or will it become illegal to sell used cars because the software license doesn't allow for transfer?

In the US? Possibly.

In the EU? Only if TTIP gets signed.

I'd search for forums for your make and model of car. Those are usually good starting places for getting information.