Most E30s are under 3000lbs, the convertible would be the heaviest. Here's Wikipedia's numbers: 2,359?3,016 lb. The 2.5L cars a fairly quick, 0-60 comes in around 7 or so seconds. The "is" models are more sporty than the standard "i."
As for the Miata vs. E30 debate, the Miata handles a bit sharper (from what thedguy has said), but obviously the E30 is more practical. It's not really that big of a difference. If you chuck the rear seats and put on lightweight wheels and put on an H&R Cup Kit (this can run any where from $1500 to $2500+ depending on the wheels and tires you choose), I'd wager that it'd be just as good if not better. I'm not sure how fast the Miatas are in a straight line, but I think they're a second or so slower than an E30 325i/is.
The 944 will be expensive. You will have to either learn how to do stuff yourself and acquire the appropriate tools or find a very good mechanic.
This website for 944 advice says every 30k miles. But with that service, other jobs are done, so the costs add up. That's why people often drive Porsches/Ferraris/etc. only on weekends, so that mileage is minimized.
EDIT: Also, with the E30 BMW 325i cars, make sure that it has a timing chain, most do, but it's still very important to check.