I would say not, unless you buy a real lemon. Then again the same is true of any car (no exceptions).
Probably stating the obvious here, but any omega you buy will have the thick end 15years under its wheels as an absolute minimum. What that means is you are well into the regular replacement cycle of parts that is needed to keep a car of that age going. Nothing lasts for ever and even non-consumable parts (rads, hoses gaskets etc) will succumb to corossion/wear given enough time.
It just depends on where in that replacement cycle your car is when you buy it as to what needs to be spent. £1200 in 6 months in new bits is a lot (for an omega), but then again its only £200/month which wouldn't buy much to ride round in on lease/pcp. I'd wager that if MigV6 kept up with that £200/month and put it into a bank account as a repairs fund he'd be well up by the end of the year, and could probably fund tax, insurance, and a good few pints on what he had left over!
The only reason it looks expensive is that newer cars take your money off you sneakily through depreciation. You don't actually hand over the cash, so it feels less painful.