Some of the latter posts have been a good attempt in deterring you from even buying one! Having previously owned a Cavalier, they rust in the similar areas - rear wheel arches, sills, and the Omega rusts on the front chassis members / rails which are partially visible from under the front wheel arches.
Having said that, the aged old advice is the same, look at as many as you can (although there won't be that many around), go for the best spec you can, and if you aren't particularly mechanically minded, take some one with you who is), read all the write ups you can on here and the internet, and when you find the right car, it will jump out at you.
2.0 and 2.2 engines are underpowered in my opinion. I have owned 2.6's, 3.0's and 3.2's V6 Omegas - they all are great to drive, the V6 sounds well, the 2.6 does perform well, but personally speaking, I prefer my 3.2's over the 3.0, both of which have the edge over the 2.6. If it is toys you are after, then the later 3.2 Elite is the King, wheras GLS and CD models have quite low spec levels.
Just do you homework, don't buy the first one you see (get the general flavour of the Omega with the first one you look at - unless it's a gem of course!). Be prepared to pay over the odds for a good example with plenty of decent history.
I've been involved with 'classic' cars for over 36 years - decent Omegas are becoming less and less, but that applies to most other cars of so called 'classic' status. One will eventually turn up if you aren't in a rush....there are good ones out there that will, at some point, come up for sale, as generally their owners are of (cough!) more senior years.....
No doubt there will be other ''helpful' advice to come.....
This a a brief but worthy read...
https://not2grand.co.uk/not-2-grand-cars/the-vauxhall-omega/