I work at
Track Dog Racing. We are a Miata Specialist shop in Dallas. If you're in Fort Worth or Denton,
Miata Solutions is a great place for service as well.
I'm a NA fan myself. I have a 1996. Some folks will tell you the 90-93 is good because they are emissions exempt at 25 years old. However, the 90-93 has the 1.6 which has less low end torque so I prefer a 94-97 with the 1.8 motor. All the NA will be emissions exempt in 6 years so it's not a big deal to me.
As for 94-95 vs. 96-97, The main difference is OBDI vs. OBDII. They both have their positives and negatives.
OBDI - Harder to troubleshoot as they tend to have fewer sensors. Also harder to get an emissions inspection as pre-OBDII inspections places are getting harder and harder to find. Also, you don't know if you will pass until you get it inspected with the tail pipe sniffer. They have fewer sensors to go wrong or monkey with if you plan on boosting it. You can run a MegaSquirt with a "clean" map to pass inspection.
OBDII - Easier to troubleshoot. Easy to pass emissions and with a OBDII scanner, you can know before hand if your car is ready to pass. More sensors means there is more to go wrong. They can be finicky if you plan on boosting it but a good shop like ours can make it run without tripping codes.
I've owned both a 1995 and a 1996 and I prefer the OBDII car myself.
My recommendation is find the cleanest car you can find in your budget. With $6k that should find a VERY clean NA. Take your time and consider opening your search wider than the immediate area. We have customers that have flown to California and Washington state and driven cars back. They got immaculate cars and had a memorable experience driving the car back home.