What Year Mustang Should I Get?

[84LX89GT]

My opinion is that for an everyday driver for comfort and also something you can throw around go with a '94-'95 mustang GT coupe for these reasons:
It has the 5.0L for which there are TONS of aftermarket parts
The Chassis is a bit more rigid than previous years and with chassis stiffening devices will be a very good platform to start with
It has 4 wheel disc brakes and ABS which will REALLY help for autocross and for crazy drivers on the road.
You're more likely to find a lower mileage model than for earlier years and it has a more smooth modern styling to it.
The 5 speed is the world class T5 used in i believe the '90+ models.
All '94-'95 cars have mass air metering systems

But it will be kind of hard to find one for in the low $7,000 dollar range.
My next recommendation would be an '89 to '93 5.0L LX, GT or '93 Mustang Cobra (i've seen some for around 7k dollars) because those models have mass air metering systems (which adapt to modifications unlike speed density systems).
A 5.0L LX coupe (a.k.a. Notchback) is the more rigid of the original "fox" platform and are lighter as well, but unfortunately are harder to find.
Good luck on your search,