Hi, firstly welcome to the mad world of FOG.
When checking the car, there are a number of things that are different from the other models.
You must first be aware that the 2.5 is completely cableless, so the ECU operates everything including the throttle.
Check for sideways play in the viscous fan hub (with engine off obviously)
This is a common fault.
Check for any obvious oil / water leaks around the engine and cylinder head.
Make sure all the levels are correct and the fluids look clean.
Does it have a service history? Should be serviced every 5000/6000 miles.
Check that it starts ok from cold.
When road testing, make sure it revs over 2000rpm when under load. If it doesn't, it will be in limp mode as there will be a fault.
Normally this has to do with the number 1 injector which has a sensor built into it, and is quite a common fault.
The engine should pull very well as they are powerful lumps.
Apart from these things, check everything you would normally check on any other vehicle.
Oh, when you start the engine, dont use the throttle, and allow to idle for a few seconds before touching the throttle.
Same when switching off, leave idling for a few seconds. This is because the turbo and the fuel pump need to be lubricated.
The oil has to get to the turbo before it winds up, and down.
The pump is lubricated by excess fuel, and it will be a very expensive mistake to forget this.
Good luck with the car, hope you get it.