Hi Chris,
The W201 190E is a very well made car, if they're looked after well they don't give much trouble and are pretty inexpensive to maintain.
I'd say the main things I would look for are:
Evidence of good maintenance, either service history and bills or similar, try and establish if it's been well maintained.
Rust - they're pretty good in this respect but there's a few places to look - jacking points, inner wings, around the rear screen/parcel shelf inside boot etc. If it's rusty, leave it, there's plenty of rust free ones out there
Engine/gearbox. Check the condition of the coolant and oil, ideally cleanish oil and relatively clean coolant (no contamination), make sure that it starts nicely from cold and idles well, shouldn't be any noise from the top end and the timing chain should be silent. Make sure it selects gear well and changes smoothly as you'd expect. Try the kickdown when it's warmed up.
Make sure everything else seems to work, eg the electric windows, sunroof (if applicable) etc. Try the central locking and all of the locks/keys. Check the general condition of the exhaust, tyres, brakes etc, this should also help to show how well it's been looked after - tyre brand, etc.
With 120K miles if it's been looked after well it should still be tight as a drum, will drive nicely and should be a great car to own.
Good luck and keep us updated how you get on
Will