nothing inherently wrong with the engine, inlets tend to get a bit gummed up, and the turbos start to go - both a result of poor sevicing, particularly infrequent oil changes. If you do get it, I would be inclined to do some quick oil changes initially over the next few months.
Engine aside, all the usual Omega things. The manual gearbox is reliable, but often replacing the oil in it does make it better in use. Front suspension is prone to wear at about 100k. Inside edge wear on front tyres is a sign camber is out (no point getting omegas tracked, need full geometry checks to be honest). Obviously, like any car, shocks can be tired after a while.
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I big issue with Omegas is clocking - the cars wear so well, and it is ever so easy to wind them back physically, it is rife. Go over the history with a fine toothed comb. If no history, get the logbook number and look up the mileage on the governments MOT page - mileage should be recorded annually since the computerised MOTs came in about 5yrs ago.