for your first car, size of the motor doesn't matter, size of the car doesn't matter, respect for the car, the elements and for those around you is what matters. People sometimes act on impulse and do something stupid and yes the faster the car the quicker the stupid impulse desicion can become catastrophy, but just think before you act!!
I've done alot of stupid things and I had alot of fun, but I had an unusally high respect for all things when I started driving and this made a big difference.
My personal picks in order would be:
'03 cobra
'03 mach1
'99+ GT
'67-'68 fastback w/o all the chrome
'93 cobra
'89-'92 lx 5.0
I wouldn't buy a '95-'98 I think they're b-tch cars, sorry, my opinion.
If I were you:
'89-'92 lx
The parts available are tremendous, they're easy to work on, they can look very good with the right body kit, they're cheap to own. Don't worry so much about the cars miles, you'll probably end up replacing the whole suspension, driveline and rebuilding the motor for horsepower in the long run, anyway. really, Mustangs are that addictive!
good luck