[Clogs]

First thing to notice is space - less of it. I had a 765 turbo, and it's wider and longer than a T5.
Second thing is acceleration... a T5 is a lot quicker with front wheel spin possible at any speed up to 60mph in lower gears. Front tyres wear quickly even if you just pootle along.
Third thing is comfort... seats in a T5 are excellent and the springs are good soaking up bumps. A bit less room in the back as well. Beware of potholes which will bend alloys as will kerbing on lower profile tyres than a 745.
Fourth thing is lack of rust for an old car - so the paintwork is usually pretty good outside and under. Look under carpets for rust and moisture and any water stains around windscreen. Make sure there aren't irritating rattles from under instrument panel (wiring touching plastic or moving), door panels, seats or tailgate.
Fifth thing is you'll have to spend a thousand or two bringing it up to scratch in the next year or two to deal with niggles - a/c compressor and/or alternator replacement, turbo seals, new cambelt/waterpump if 90k, new discs and pads, fix tailgate rattle (new clips available on internet)... but that is to be expected on any old car.
Sixth - invest in a Haynes manual and see this section for a wealth of advice.
Seventhly - I get 27-29 mpg - only 32-33mpg on long motorway journeys, but then the 765 only did low to mid 20s lugging a high tonnage of metal. Always use high octane petrol... mine doesn't like basic 4 star at higher speeds.

Lastly make sure absolutely everything works and all lights (incl instrument panel), air con, cruise control, radio/tape player, auto interior fans at all settings (make sure really hot and really cold), seat catches, seat adjustment works. The radiator water should be clear - any sight of the dreaded stop leak to be avoided as that will cause trouble later - new rad/heater matrix and 3 o'clock temperature gauge when warm (after less than a mile from cold). Also all windows wind up and down from each door and from driver's door, the lights at the bottom back of each door work and don't stay on to drain the battery, neither does the light in the glovebox not go out. Wiper washers on headlights should work as should fog lights (front and rear), and headlights move up or down. No oil leaks should be present and no blue smoke in hard acceleration or after coasting down a long hill - both turbo wear signs - expensive! There should be no blow back if you take the oil filler cap off - it should suck a little and there should be no gunge in the oil dip stick or oil filler cap.

T5s are more fun to drive than earlier Volvos and look good as well, better than later Volvos.