I am only recently an Omega convert, have a 2.2 nearing 100k which has had the usual issues at this age/mileage-has been through workshop for the following (may be useful to look out for/negotiate price with when you look at cars):
Major service with plugs, pollen filter etc
Engine flush, new oil and filters
Rocker cover gasket replaced
New discs/pads all round
Swapped tyres around/balanced/tracked-was tramlining
Replaced wiper blades
Replaced tatty remote key outers and batteries
Tech2 read and ECU/box update at VX
Cruise stalk fitted and enabled
Replaced fuel release motor
Replaced horn
Greased everything, sorted a few squeaks
Noisy door strap washer replaced
Valet/t-cut/polish/Rain-x
Some minor dings/bumper marks cut and touched in
Now getting there and being assisted by helpful forum member this weekend to do the last bits;
Replace headlight inners/whole units-adjusters bust
Another short oil change to cure top end rattle
Coolant change
ATF/gasket change
Boot release mod
It has taken some work but is now coming together nicely-I would recommend a decent refurb budget if you want it "right" and would say you would do well to ask advice on here re: buying before you part with the cash.
Happy hunting

PM if you have any 2.2 specific queries-I'll do my best to help.