Look for 7/8/9/10 speed cassette hub, indexing, shimano; Exage, 105, 600, Ultegra or maybe a current- 1 generation old Tiagra group.
Shimano 'hyperglide' ramps on the cassette, and ramps and pins on the Cranks came in in the late 80s and was standard by the early 90s on indexing groups. That's where the good shifting comes from.
Ignore bikes with stem shifters, threaded freewheel hubs and rear friction shifting.
Indexed 7/8 speed groups with downtube/ bar end shifters and hyperglide cassettes are a better bet than early STI shifters IMO.
Dual pivot brakes are another thing to look for to find a modern bike from the 90s.
A shimano 8/9 drivetrain gives you a few options for running campagnolo brifters too (which are $100-$200 a set new).