Doug, if it's not already too late, also check out Tire Rack. Their site allows you to search for your specific year and model based on several criteria, including weight. Generally speaking, and all else being equal, the lighter your wheels, the better your tires grip the road and the better your car handles. That's what a wheel upgrade should get you. Look up "unsprung weight" to learn more about this.
The big question is, what do you want to do with your car?
The link below should take you to a search for wheels to fit your 1990 NA based on 15" diameter and 6", 6.5" and 7" width, and sorted by weight, lightest to heaviest.
https://www.tirerack.com/wheels/results.jsp?&autoMake=Mazda&autoModel=Miata&autoYear=1990&autoModClar=
For sporty driving on public roads, these widths, and the tires that match them properly, should fit under your fenders without modification and not interfere with brakes or suspension. 15X6 is a moderate upgrade from stock, originally offered by the factory, and will fit your NA very well. I run factory 15X6s on my supercharged NB road car with 195/50-15 summer performance tires, and I like the light, sensitive feel, but then I don't drive it flat out.
Sorting by weight shows that a light wheel is about 10 to 11 lb and will cost about $200 to $250 per wheel at Tire Rack. It also shows that higher price doesn't guarantee lightness - some of the heaviest wheels, at about 19 lb, still cost over $200. My factory 15X6s weigh about 13 lb and cost less than anything Tire Rack sells, so use that as a benchmark.
If you're going to autocross or track your car, check the regulations for the class you want to compete in, find out what the leaders in that class are running, and choose accordingly. This is where Flyin' Miata and similar vendors come in, they carry a lot of gear that can be used on road cars, but mainly emphasize performance for competition and the prices reflect that.
None of the above has anything to do with appearance. If appearance is a priority, then it will probably cost more and could very well compromise handling quality and performance.
As for tires, look at the UTQG number, the rubber's treadwear rating. The lower, the stickier. Most four-season radials are about 500. Summer performance tires are about 200-300 and give an enjoyable increase in traction on warm, dry roads once they're warmed up, but shouldn't be driven below about 40 degrees and are dangerous on snow and ice. Track tires are about 100 and, like track brakes, are at their best at the sustained high temperatures of flat-out driving, so not a good choice for public roads.
Good luck, have fun, and get home safe!
Del Val Miata Club