Hi Lee,
1K should get you a mint, well looked after pre-2000 model year 3.0, or a good selection of elite/CDX 2.5s as there are much more of them about so they tend the be the real bargains.
However, these V6 engines are quite complex to work on, so if you pick an iffy one you can easily spend all your money again on having it fixed, unless of course you like to do DIY, in which case it won't be so much of an issue, although I would also warn you that parts prices from vauxhall are far from cheap (I would even go as far as to say that BMW are better overall on value!!
).
So, check points:
The light lenses you have noticed and yes, they are a common issue on pre-facelift omegas.
Rust is also a bit of an issue on older omegas, the doors and rear arches being the weak spots, so check for those. Otherwise they are generally pretty solid.
As I said earlier, the engines are generally tough mechanically, but even minor jobs such as the cam cover seals or spark plugs can be quite time consuming (and hence expensive if taken to a garage). Check for oil leaks from around the edges of the cam covers, and around the sump. A burning oil smell when warming up or oily sludge on the undertray are dead givaways there. Also run the engine at idle for a while untill the fans kick in, and make sure the temp stays between 92 and 98C on the guage. A car with history that has had regular oil changes is worth paying extra for. Also, knowing when the cambelt was last done is good as that is a 600 quid job at least!
Suspension is a wear and tear item and will become soft with age. This is not so expensive to fix and is comparitively simple DIY. So long as the tyres arent badly worn on the edges and there is no clonking/knocking (worn bushes) when driving I would say its probably normal for a 10 year old car to be a little soft.
As you are looking for a model with all the kit, make sure all the kit works too! Otherwise thats just more work/money for you.
With things like trim and spare parts, there are plenty of breakers on here that will be able to supply you with what you are looking for at a bargain price, and of course there is ebay too, so there hopefully won't be much need for you to visit the vauxhall stealership too often.
Also, have you considered the Omega 2.5 TD with the BMW straight 6 engine in it?
They are quite tuneable, very robust and pretty economical too