If its on a 03 plate, it will be a 2.6
Some of the early 2.6's suffered with valve stem failure.
Look for blue smoke on start up
Gearboxes are pretty much bomb proof imo.
If the car is 13 years old, i'd change all fluids and filters inc the gearbox one.
The gearbox is easy enough if you can get it up on 4 ramps / tall stands but messy.
Cam belt / tensioner should be changed due to its age. These are very temperamental to cam timing and unless you have the FULL lock off kit, its impossible to do properly regardless of what any garage will tell you
They are lads on here who can do that job for you properly for appropriate exchange of sheckles or if the buy the bits yourself, the forum has a DVD which is spot on for any level of mechanic
Other then that, just make sure most or everything is working and listen for any taps or rattles that don't sound too healthy
But the main thing is ............... don't be afraid to walk away if it doesn't feel right
Good luck