Hi,
It's a bit hard to predict, especially when you have limited knowledge of a car's history, but TRs are really quite dependable. And, when something goes wrong it's usually an easy repair. I've re-adjusted a slipped distributor in the rain, in the dark, with only a Bic lighter for illumination, and got home okay.
Take along a repair manual, if you have one. It can come in really handy if you have a problem and take the car to a mechanic, but he doesn't know the specs of a TR6.
Check the spare for air, that the lug wrench fits the lug nuts and that the jack works! These sound silly, but I've "been there, done that"!
I also usually carry a small briefcase style toolbox with basic wrenches, screwdrivers, pliers, etc. Also spare fuses, replacment light bulbs, timing light, spare points, extra spark plugs and a spare fan belt. It also has a flashlight, a rag or two and a couple pairs of latex gloves. Some electrical and duct tape can be handy to have, too.
I wouldn't bother with a thermostat, unless you really feel you must. You can buy one almost anywhere that will do well enough to get you home. Maybe a fuel pump repair kit would be a good idea, take up less space than a complete pump.
After that, I'd only think in terms of anything I feel would be really essential and difficult to find in a parts store along the way. Only things that you and the car can't get along without temporarily.
For example, a manually shifted car can be push-started easily, if a starter fails. Just be sure to park at the top of a hill, rather than the bottom.
Or, most shops have alternators that can be fitted, if one fails on a trip.
Even most radiator hoses can be found, or flexible "universal" type can be temporarily fitted to get you home.
Besides that, I don't know if you have changed all the fluids, lubed everything and spent some time closely inspecting brake lines, pads and shoes. Also, look over fuel lines, and inspect for leaking oil at the diff, gearbox and sump. I generally change all the hoses and belts on a car when I first get it and don't know it's history.
Then, have a great trip!
Cheers!
Alan