My advice: you should figure out what you really want to do first.
Do you just want to enjoy the occasional open lapping day with friends? If so, pick whatever you like! Drive everything that interests you and buy whatever tickles your fancy.
Do you actually want to compete and be competitive? If so, figure out what class you would like to eventually run in and pick a competitive car. For time attack, the most popular series are Gridlife, NASA TT and SCCA TT is also starting to pick up. They all have their pros and cons. Some of Gridlife's more popular series are things like Club TR, which pretty much means to be competitive, you're probably going to need something with a K24 swap... NASA's classes are pretty much all power-to-weight ratio based- leaving you with plenty of options on what to drive, but you need to find the appropriate PWR you're looking for and read the rules to build the right car... SCCA's TT classes are 100% centered around "the car to have", which is usually the most popular car in their respective classes. SCCA builds can be cheaper in lower classes (more restrictive rulesets), but you truly need to have the right car, which sometimes changes ever year or 2. Pick your poison.
I'm not sure where you're located, but if you want to compete locally, you should looks into local leagues. No point in building a car for XYZ class, only to discover that no one locally competes in that class... I've been in a class of 1 before, trust me, it's no fun.
You should probably understand how much it truly costs to build a competitive time attack car... the last time I decided I was going to attempt to build a competitive time attack car, I was building it to run NASA TT6, which is the cheapest, lowest powered class that NASA has. I was building a car that was probably worth ~$10,000. By the time the basic bones of the car was built, I had invested an additional $20,000 worth of parts into the car... for a car making ~160whp... that's with me doing all of my own wrenching. No aero, nothing crazy. That's for a cheap car, in a "cheap" class. I don't mean to be insulting, but have you looked into how much it would cost to build a competitive time attack B58 powered Supra or 240i? Buying a car is only the tip of the iceburg as far as expenses are concerned.
The cheapest way to get into motorsports (by far) is autocross. However, your seat time is very limited. It's a great way to build technical skill and to meet friendly folks that will likely help you to get faster, but it can be a bit of a tough sell- get up early, drive far away, work the track for hours standing in the sun for 3-6 runs that are only 40-60 seconds long. Unless you love it, it can be draining. With that said, what's great about autocross is that most of the popular Street SCCA classes have restrictive rulesets, so not much money is spent on mods to be competitive. And by comparison, it's not very hard on your car. For time attack, there's quite a bit to consider:
-Keeping coolant and oil temps down for 20+ minute long lapping sessions. This frequently means bigger radiator, some type of oil cooler, sometimes ducting, vented hoods, etc.
-In time attack, depending on your platform, you can eat through tires and brakes pretty quickly. This can add up to your season costs to run. To save on consumables, entering a lower power class and keeping the weight down can save you quite a bit of money.
-Safety. How fast is your car on track? In the event of a roll over or high speed collision, will your street car save your life? Cages aren't exactly cheap and they don't exactly help with resale value...
-Running a car hard for a long period of time sometimes results in blowing a drivetrain component... the same can be said for the dreaded "money shift" on a manual car... can you afford to replace a blown motor on track? For an older Miata, that might be a $500-$4000 affair (plus labor if you can't swap it yourself). For something more exotic or harder to come by, you could be looking at a much larger bill.
-Can you afford to walk away from a $60k car that you put into a wall at turn 4 when you misjudged your entry speed? Keep in mind that most of the time, your street car insurance will not cover you for timed events... you can purchase track day insurance, but it's not cheap and will add up if you plan on running for an entire season.
-What does your tool set look like? How mechanically inclined are you? Things break on track. Frequently. Paying someone else isn't cheap... but neither are tools. I would imagine many of the folks on this forums have invested tens of thousands of dollars in tools over the years (I know I have- and I'm a novice compared to some of these nuts!).
^^^Many of those considerations go away with autocross, or are at least drastically diminished as autocross isn't nearly as hard on your car as time attack- much shorter runs (~60 seconds vs 20+ minutes) with much lower speeds (typically no faster than ~60mph) and far less of a chance of a money shift (in most cars, depending on gearing, you shift into 2nd towards the beginning of the run and just remain in 2nd).
So, again, do some research, get out there and figure out what sounds most appealing to you. If you want to compete, find the most competitive cars in the class (classes?) you're most interested in and buy that. For competition, there's no real blanket answer, as it's more class dependant than anything.
If you just want to do open lapping days with friends, while learning the basics of car control in the real world, heed the advice of everyone on this forum: Miata Is Always The Answer. Although I would go against the grain here and instead of recommending an NA/NB or ND, I would pick up an NC. These days, NC's are similar in price to the NA/NB, but with more chassis stiffness, more interior space for taller drivers, quite a bit more acceleration, they're much newer than the NA/NB (read: less maintenance and leaks), don't have the weaker tranny of the ND, 2.5L swaps are cheap and if you ever wanted to up your power game with NA power, the fine folks at KPower are FINALLY releasing a K-swap kit for the NC: https://kpower.industries/
^^^I have to stop typing to avoid buying another NC and lusting after a K24 swap for it.
