Whatever you decide on the S80, you keep that '93 240. You won't find a most reliable volvo than the '93 240, some volvo loyals call it the "best" volvo ever built.

Having said that, $8500 for a 2000 S80 with a rebuilt tranny sounds like a good deal, and you know what you're getting into. The S80 is a dealer intensive expensive maintenance vehicle with lots of electronic gizmos and computer modules, of which only the dealer can service, read the codes, upgrade software etc. Expect routine service from the dealer at minimum $400 each time, not including parts, just for diagnosis, clearing codes, etc.
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'88 240, '92 745, '98 v70 John, Tampa Bay