Whilst Ryan is correct, any car can be looking after and run for a long time, but 9 times out of 10, a saxo with high milage is on its way out...
If you can afford the higher insurance / tax of the VTS, then get one. they are far far better than a vtr.
running costs aren't that different. you can acheive decent MPG in a vts, i regularly had a similar amount of miles from a tank as i did in my vtr.
common problems include:
buggered rear beam - you can see negative camber on the rear wheels
rust in the engine bay - you can see it, usually under the ECU.
rusty rear arches
buggered gear box.
buggered engine, a VTS should pull well beyond 3k revs.
body panels not lining up.
the vast majority of saxos will have a load of simple problems with them, sloppy gear linkages, abs light on etc. these kind of things, whilst annoying are easily fixable.
If the handbrake is quite high / not centre when on, it's likley to have had some abuse
try and find one with service history too.