I used to own a 90 convertible and have a 95 convertible now - I can help you out with some of these questions.
I'm not exactly sure on the weight - someone will have to chime in on that one.
They are capable of running in the 14s stock if you have a stick and can drive it. The autotragics are a bit slower - I went 15.3 w/ a 2.4 60' with a K&N and pullies on my AODE.
The convertible top seals MUCH better on the SN bodystyle and overall there are much fewer squeaks and rattles. Get some subframes and you will be even more amazed.
Maintenance wise it is about the same stuff, you've still got the same 5.0 engine and not a *whole* lot was changed. The steering racks on the SN's don't have the problems that the Foxes have, the TFI module got relocated so it doesn't cause problems nearly as much. You won't have to mess with the TPS.
You will get disc brakes and 5lugs all the way around on a SN.
The computer is not as forgiving on mods as the fox bodies are. You'll most likely need a chip or tuner when you go beyond bolt ons for your mods. (cam etc) This isn't that big of a deal though, don't let it put you off.
I really like my 95 - prefer it over the 90 actually. When you're looking for one, look good at the top and check for leaks - that will be around $1k to replace if it needs replacing so keep that in mind.