The stock V6 power in the UK is supposed to be 161 bhp. The difference will obviously depend to some extent on which V6 Vectra you had, as they weren't all the same!
The first thing I would do with it is give it a bit of a service. To me, this would involve: changing the plugs for some NGK Iridiums (BKR6EIX-11), changing the air filter, and changing the oil and filter (using a good synthetic oil), as a minimum. I would also consider changing the fuel filter, gearbox oil, and possibly the coolant. A lot of this depends on how long you're intending to keep the car though, as I would also be looking at things like cambelt & water pump.
Also, one of the easiest upgrades for the V6 is to fit a VR-4 strut brace. It bolts straight on, and will transform the handling.