Need some advice..

[84LX89GT]

If you're looking to buy something to tear down and build up from the ground as a drag race car a 4 banger mustang is the way to go.
If you're looking for a street machine my opinion is that a '89 to '93 Mustang 5.0 is your best bet. This is because it already has the better braking, suspension, cooling, and heavy duty drivetrain and go 14's stock. Alot of people would say don't go for '93's because they have hypereutectic pistons rather than forged, but it really doesn't matter since you want to go naturally aspirated.
Also the '89 to '93 stangs has the mass air air metering system which adapts much better to modifications than the speed density.
The fastest mustangs go from LX coupe, LX hatchback, GT hatchback, LX convertible, GT convertible because of their weight and rigidity.
5 speed mustangs are from what i hear about .5 seconds faster in the 1/4 mile (considering everything equal stock) than the automatic mustangs.
i believe the '90+ 5.0L 5 speeds have the higher torque rated "world class T-5" transmission which has a higher torque rating than the non-world class transmissions (put up to more abuse before you need to upgrade).

I got my '89 for $1500 total and was already partially modified and in decent shape so you should be able to find a good deal.