You can DIY a lot of the mechanical things, the problem is that a lot of the electrical stuff is 'software locked' to the car so you can't just pull it from the junkyard. That said, if you get an 04+ car, consider getting a DICE apparatus. It usually comes with Vadis (amazing software that tells you all about your car) and will let you see Volvo codes. It really helps diagnose your problems if any arise. The big downside is that they run about $500, a lot for an OBD scanner (though it does more than just read codes..)

Anyway, have you driven an S40? If not, then do it. Also, try an s60, they're a bit roomier and have more power. They tend to be priced about the same.

If you stay away from first years, that means stay away from '05 S40s. I haven't heard bad things about them in particular (like the '99 S/V70s) but they're only 5-6 years old, so we'll see.
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If you're not driving it "like its stolen," are you really driving?