I would spend as little as possible and not make it a goal to borrow or spend that much unless you find a screamin deal on a very well done car. If it's gonna be a DD though, I think you got too high of a budget for a TR. No use buying one that's worth more than 8-12k if it's going to get run down. NOT saying you won't take care of it, but the sun takes it's toll on the plastic stuff pretty fast when it's this old. Add to that the parking lot rash, potential dickheads with keys, etc. I'd honestly buy a 6-8k t type that's a light color and presents ok but not so nice that you worry about it. Trust me you will enjoy it more. Light colors stay clean easier and it's easier to hide the flaws to the casual observer. Also don't buy one with t tops. They're ok on a weekend cruiser but day in/day out they'll be pissing you off in less than a week. There's lots of modern stuff out there to make these cars pretty reliable as long as it's not subjected to detonation. The powermaster brakes can be converted, the maf upgraded, bailey's got a new CNP coil/module comin out, etc.
If you show a turbo buick a picture of a salted icy road, it will instantly rust clear up to the door handles. Might also invest in a winter beater/backup vehicle.
