i'll take a stab at a few of those questions:

any GLTs before 1997 were just Normally Aspired 168hp engines. 1997 GLTs have the low pressure turbo, producing around 190hp.

the T-5 and Turbo are standard turbo models, while the T-5R and R models are upgraded turbo models fully loaded, with extras such as suede seat parts and 17" rims.

so it depends.. if you want the authenticity of a T-5R or R and their associated 17" alloy wheels and a few other goodies.. go for it. otherwise a regular turbo can be just as fast in the end with the right mods

the V70 (wagon) and S70 (sedan) aren't a whole lot different from the 850.. some body differences and a few other changes

as for the turbo question, i don't know.

a few things to be aware of while buying: check to make sure the A/C works, and that there are no ABS lights on. those are some of the main things to go wrong with 850s