As Ken says, it's all covered in the guides...
And all depends on asking price, budget, repair budget, your approach/mindset, how much you're prepared to do (or not) and ultimately gut feeling.
Omegas only have a handful of common faults and these are well documented.
Any Omega, regardless of purchase price is likely to need around a grand spent on it in the first year, and that's just suspension, brakes, cambelt and geometry and assumes that you do the work yourself. Even a half decent service will cost £120 in bits.
Basically, if you like the look of the car, buy it. Any doubts, don't.
If you really like the car, you might wish to remove the sill covers and empty the boot out/remove rear seat base and pull rear carpet forward... Just to be sure...
Just remember, any decision is made using YOUR subjectivity, and anything we say is largely immaterial in this regard