Buick and Olds shared the same 215 cubic inch engine, which is essentially the same as the Rover 3.5. There are some differences between the "old" GM and the Rover, mainly in the ancillary bits.
Whether it will fit in your TR7, however, depends on the manifold and carb setup. There's little clearance available, and it takes a low profile manifold/carb/air filter to fit.
The biggest hurdle, however, is the engine subframe. The TR7 subframe won't work, and will have to be replaced with a TR8 subframe, or adapted. You'll also need the proper bellhousing, drive shaft, radiator, etc.
Lots of other issues to attend to, but overall worth it if you have the V8 bug. As for value, it's more about what you want out of it, but a transplant done properly only increases a Wedge's desirability. (Unless it's a Spider, or a museum piece.) Since it sounds like you're more interested in the TR3 anyway, selling it like it is sounds like where you're headed.