It's a very good idea to do the belt and tensioners as soon as you get the car. You know where you stand then. If you're going to do that anyway, it's less important that the seller has had it done recently, but of course it's a good bargaining point.
The 2.6 is no better or worse than the rest of the V6 models so what you read about them applies to all cars, really. The 2.6 and 3.2 are later engines that have a few differences from the previous 2.5 / 3.0 engines. They have a drive by wire throttle, so no throttle cable, the secondary air injection and exhaust gas recirculation systems are gone, they have a coil-per-plug ignition system instead of plug leads and a coil pack. There are pro's and con's to all of this and the net result is probably about neutral.
As someone who came to Omegas only about 9 months ago but having previously maintained my own cars. I'd say the Omega isn't a bad car to work on at all. The routine maintenance tasks are all very straightforward. Don't be put off by the lack of a Haynes manual. This place is much better. There are one or two relatively minor faults (you'll have seen them mentioned here

) that occur frequently but other than that, keep up regular oil and coolant changes, keep on top of the cam belt and tensioners and you can't go wrong.
Kevin