Check the selespeed fluid, should be clear, if its black, its got oil in it, not good.
Ask if the seller has topped up, or the garage has, and what they use (should be Tutela CS Speed)
Use all the gears in both city and manual mode.
If you have the key set to MAR, you should be able to cycle though all the gears with the engine off, if I am not mistaken (Dont know 147s).
Ask if he has had any problems with it in the past.
The Selespeed have had some problems, but its not as bad as everyone made it out to be.
Often when something breaks it could be a pump, or tank, or even the ECU
Most dealers want to blame it on the system when they are unable to find the flaw, and their only solution is to replace the whole lot (a 1000 quid job)
And if you ever want to get rid of the 17" Selespeed alloys, send me a PM