I personally Think the
280 (especially when its been mapped) is one of the best cars over time.
But with regards to issues with the car as everybody has said you need to check the service record.
But this does not show you how the car has been driven. For example my car spends 90% of its time on the race track so the service record looks fantastic (Oil change approx 1 - 2 months, brake fluid change Yearly to 18 months, dsg service 2 yearly etc etc) but the car has been driven hard.
Now I will never sell my motor but my point is service record on its own does not tell you all.
So if the front diff has not been done then you need to factor that into the cost of purchase.
If the Water Pump has not been changed then you know over time it will need doing so again factor that into the cost of purchase.
There is also a big issue on VAG motors with the cooling system blocking (see the forum) again if the matrix has not been replaced it may need to be done so again allow for cost of change.
This is the same issue with any car you buy and lets face it Brand new VAG Motors are full of bugs but you are covered (hopefully) by the warranty so check what is covered by the warranty.
To buy any car you need to drive it, listen out for rattles (Seat are famous for them) and suspension noises, some of this can be sorted with sound deadening cladding but some mean there is other stuff going on but you wont know till you have driven it.
DSG Box on these cars are generally good but if they go its expensive (as they are on the VW Golf etc) so again a drive will tell you if it feels good.
As always with a purchase use every bit of kit the car has to make sure they all work.
Front light covers can get misty Seat say this does not impact performance but can look odd so give them a check and make sure you are happy.
Check the Boot well to see if there is any rust (leaks can come through the rear lights and the air flaps behind the rear bumper, again does not stop you buying the motor but does require some work to stop further problems.
If you have an
OBD Scan tool then run a diagnostic test to a) check millage and b) see if there are any errors shown.
If the car has cruise control make sure it works correctly (the front sensor is not cheap to replace).
Personally on my motor the issues I have had are mainly around silly things like the Petrol Flap being locked (solenoid needed to be replaced) but I used
coding tools to turn off the auto lock when driving to reduce the number of interaction with it going forward.
ABS Speed sensors I have had a few, is suspect more than most due to how the car is used
EVAP failure - this is a cheap replacement part.
The tuning opportunities on this car are endless but do your research and think about what you want to get out of the car before you start.
Hope that helps