Figure out what class you want to run, how available it is in the area, and if you argee with the rules and class philosophy. I have heard a couple people not liking the Spec Miata class philosophy and chose to modify thier car for Improved Touring.
IMO Spec Series are the only way to go for drivers (damn being stuck with this current car or damn the lack of support for SE-R cup). SCCA is stronger in the North East than NASA (more races, but if you are only going to do a few a year NASA is more than fine).
I have only thumbed through the rules but seems relatively cheap. http://www.nasaproracing.com/rules/SpecE30.pdf
Although the E30 vs. E36 is getting close for the initial price of the car again read the rule set. How much are tires is the allowable sizes, what are the allowable engine modifications, what are the allowable suspension upgrades. “Saving” on buying a “fresher” E36 could cost much more in the long run.
I am weak on my PCA , BMWCCA rules so google it.
Lastly, dollar for dollar the smart thing would be Formula Vee or Formula 500. Old VW or 500cc snowmobile powered formula styled cars with skinny tires or wider tires with F500. @ ~500 lbs you can tow it with anything, are in a spec series, have something that barely get the tires warm (easy on equipment) and the competition is great.