I've got a '98 'vert, and it's been pretty good in the winter. I believe they have the same top, lining and gaskets, so yours should be pretty good as well. Pull the rear seat bottom and look to see if there's any rust on the floor. That will tell you if the top is leaking. Also, look in the trunk well under the spare tire.
You also want to look for horizontal wear spots about 6 inches behind the rear quarter window. If there is any wear there at all (in the "sail panel", not on the front edge of the seam of the top), you'll need a new top within the year. Also look at the rear window. The vinyl is glued and heat sealed to the glass. If you see any indication that the vinyl is pulling away, you'll need a new window within the year. Negotiate well on either topic. A new top alone is ~ $6oo, window alone is $500, together they're a G note.
Take it to the car wash and spray the p!$$ out of the windows and gaskets, look for leaks. You'll get a drop here and there, but there shouldn't be anything major. Feel the carpets afterward, as well, for moisture. The good news is that even if the weatherstrips are bad, they're still available from Ford, and pricey but not unbearable.
I believe yours should have a liner in the top just like mine. Overall these tops seal pretty well, and keep heat in and cold out (and vice versa). I'm always warm in the winter, and cool in the summer, even with a black car. You'll want to be sure the AC works, BTW. Everybody says, "Hey, it's a convertible. Just put the top down." WRONG!!! You'll want the air, trust me.
The 94 / 94's had the 5.0, so the motor is bullet proof, and performance parts are cheap, but they did have some unique parts and issues. Look at a Texas Mustang parts catalog (or your vendor of choice). You'll see some of what I'm talking about.
Everyone also asks "How's that Mustang in the winter (nudge, nudge, wink, wink)?" I always answer, "Actually, it's fine. It's better than my FWD SHO was, because it has a Trac-Lok diff." That always glazes their eyes over, because of course they bought the car maker marketing BS about FWD superiority in the winter. Get some snow tires, and pass every effing idiot Camry driver in North America.
It was a dissertation, but there you have it.
