Boxster as best track car under 15K
 

I agree with the Boxster, Miata, and MazdaSpeed 3.

Not sure about the Audi station wagon.

But a definite no on the Triumph. I had a 1976 Spitfire 1500 and it was the most unreliable car I ever owned (except for the Triumph TR-6 that I replaced it with).

That $6900 zanzi red base 986 is still available...
Either a steal or needs engine work
123k miles is not a lot for a 2.7

MazdaSpeed 3. Can you spell "torque steer"? I definitely agree on the Miata and Boxster!

I've gotta say if your giving yourself a 15 margin you could get a much cleaner and track ready boxster within the margin; as for my opinion on the other vehicles, I vote neigh on the Mazda 3, that FWD stuff is blasphemy, and the triumph would make a good project car but likely not track car.

I would add two of my other favorite Porsches. 944 S2/968 or a 944 turbo. I'm not a track guy but know a lot that do.

Here's a video of Wheelers and Dealers. Two episodes. The Boxster crowd may not appreciate as much since it's not Boxster related.
They purchase a 944 turbo for 2800 pounds and spend about 2000 pounds on Suspension, Roll cage, Chips and track wheels and turn this into a track car. The chips alone increased WHP from 230 to 261 WHP. The 944 turbo has the balance and weight of the Boxster but they have more power. Too ad they didn't add a test pipe in place of the cat.

https://www.youtube.com/watch?v=s1jDZKdwKGI
https://www.youtube.com/watch?v=lLEJSBOABy0 Part 2

They forgot to change the brake fluid! ;-). Cool video, thanks for posting!

Sorry, but for $15k, you could buy a decent E36 M3 AND have money left over for maintaince and suspension mods and it will that will be WAY more affordable and capable than a Boxster (or any Porsche for the same money).

Sorry but I disagree with you. The E36 M3 is a great car but for $15k you can get a decent modified 944 turbo that is already track ready. It is lighter by 400 lbs (stock), well balanced and has more power than the E36 M3. It's also much easier to add more HP than the M3.
Take a look at what a modified 944 turbo can do on a track. The car has 180,000 miles.
https://www.youtube.com/watch?v=wge2iLnfPgo

When I was younger and stupid I use to pick on them on the street.

Forced induction and track car are two things that can rarely exist well together in the long term. (and have fun rebuilding that Alusil engine) If you're a 944 (or 951) fan, then there is likely anything I can say to change your mind (they seem to be a fanatical bunch), but I've been in the "track car" industry for a long time and aside from the 944 spec class, there isn't really anything that the 944 chassis does especially well. The BMW platform is way more versitile and the aftermarket support is far superior to the 944, not to mention the rear suspension design on the 944 is more similar to a VW type I, than a modern race car.

Saying that any Porsche aside from maybe Boxster (maybe) is a good track car "for the money" really needs to come with the disclaimer that not only will you need to be doing all your own driving but also all you own wrenching. And that's assuming you can manage the cost of parts in addition to all your other consumables.

Miata FTW by a long way. You'll become the same experienced driver, with all the same smiles with money still left in your pocket.

Miatas are good for what they are, but I think the BMW chassis allows more room to grow, and if you live in a region that doesn't have a good Spec Miata field, there really isn't much you can do with the car after you go beyond DE. Plus, if you are over 5'10", Miata's are really too small to be safe IMO.