agreed on newest model advice..
Omegas are cheap to buy but when new parts become necessary the total cost can be non-economical so check the condition of the car very detailed..
here is some short check list (you can find some others from the net also):
* the tires , brand, thread depth , production date , unusual wears (means some irregularity in front-rear geometry) new good tires are expensive..
* the air conditioning system , heating and cooling properly .. and no irregular noises..
* the paint condition (under some light) for repairs , scratches, non-matching colors or some bad paint job..
* the gaps between body components equal and normal.. check the gap of all tires to bumpers ..
* road computer for dead pixels
* electrical components like windows, cd-player , sunroof, rear curtain etc.
* no EML (orange-yellow) light must be existent on the dash..
* Engine (most complex part to check)
listen to engine while its idling starting from cold (must) , a fine idle with no irregular -unusual sounds (except from hydraulic tappet noise

) is a good candidate to buy.. check the coolant and oil condition ..
* gearbox - depends on type.. (best check is to try it on the road)
* condition of bushes both at front and rear..
* also check under the car for exhaust parts , oil and other leaks , and some extra repair traces..
and definitely try it on the road..
good luck