IT is great because used race cars are so cheap for that class. The cars are reliable for the most part and for dollars and track time you can't go wrong. If you just want to get on the track and go racing, then IT is for you.
It seems you want a little more than that, and are looking more for a little more. As great as IT is, it is a little hard to get excited over the myriad of Mazda's, VW's, etc.
Vintage racing can be done on the cheap, but like IT, to run at the front of the bigger groups is going to require quite the effort in prep. I've seen guys out there in MG's with roll bars, a fuel cell and just general safety equipment. If you show up like this, most clubs don't seem to mind dropping you down to a slower run group, and a lot of times you'll still find cars to race with. If you show up with an IT prepped sort of car, ie: gutted interior, full cage, uprated suspension, and a mildly built motor, you'll be running mid pack and you will be involved in some good clean racing.
To run at and near the front, you'll need a full prep motor, close ratio's gearbox, a very sorted suspension set-up, along with driving experience (some of these guys have been racing for 35+ years).
Get in contact with TR6 racers, and they will tell you what you need as far as the driveline is concerned. If you already have the car, and will be doing most of the wrenching yourself, I would think you could run in the middle of the pack with about a $12,000 investment (remember these older cars have problem area's that need to be addressed meaning a more dollars)... but then again, you could have bought at least two IT cars for that price.