AMC Rules.
I've been going through a similar process this year. I started doing open track days again after a 10yr hiatus. I did the 3 earlier this year in my 2003 z06 vette and then, in May, did a 2 day event at Buttonwillow with my '73 street Javelin. I've since done a 3rd track day with the Javelin at Thunderhill and am scheduled for another 5 tracks days this year (all with the Javelin). once I ran the javelin I was hooked. even though the z06 has a hundred more horse power it's just so much more satisfying to be out there with my AMC. If you're an AMC guy you need to build an AMC. notchback stangs are great and make great cars for anything (street, track, race, drag)... but it's not an AMC.
After that first weekend with the Javelin I realized I wanted a dedicated track car and it needed to be a Javelin. I then quickly realized that if I was going to make a track car I should make it eligible for some racing series so if I decided I wanted to race I'd have a home. That quickly evolved into the realization that I wanted to build a RACE car that I could run at open track days for more seat time...
As has been accurately stated there's a big difference between a track day car and a full race car. I looked around (including SVRA) and decided to build for NASA American Iron. SVRA is very "period" based which I think makes things more expensive. They also have a 1972 cut off and the shell I'm building is a '74. But the biggest issue for me with SVRA is they didn't have very many events in California and I'm not going to be traveling all of the country to go to races.
If you just want a track day car I think you could easily build a solid, fun, reliable car for $10k. I've been really surprised by how well my street Javelin works on track. Now, it's not a stocker but the things that make it track worthy are: big wilwood brake kit ($2k), coil over suspension ($700), 16x8 vintage wheelworks wheels with kumho victoracer dot legal track tires and a power steering cooler. that's really it. It's got a 360 with headers and was rebuilt with a cam and a little head work. It's been dyno'd with 267hp at the rear wells. so this is a tame motor. but it still get's up and goes. it never runs hot and I drive it to all the events I go to, beat on it, then drive it home. If I was going to run this car long term it would need a road race pan but for the next few events I'll just put in an extra quart of oil.
A race car ups the ante for sure. anyway, just wanted to put in a strong vote for AMC. We need more cars on track. I'm always surprised by how much of a stir my Javelin makes when I bring it to track days. Good luck with whatever you decide.
I've got some in-car track videos on my blog that might inspire you to go for the AMC:
http://asifnyc.com