Hi there, I've just recently bought my Omega, and would agree with pretty much all of the advice - I'd also add to make sure it's got the right tyres fitted (unlike mine!).
I've bought an auto (because I prefer them), and there are a lot of horror stories on here, but I'm sure that there are many others who have gearboxes that are just fine - I think there is a bit of luck involved as with any car over about 5 years old - by picking one with a decent service history, I guess you are minimising the risk.
My last car was a Fiat Marea, and if you look at them on the Fiat forum, there are loads of issues - having said that, I had mine for three years, and it was the most reliable car I ever owned, with well over 100k on the clock, and my father in law has it now - as I say, there are plenty of bad cars, but also plenty of good ones.
Good luck in your search

!