Make sure you take it for a long test drive and understand the negatives of gearboxes like this. For example, slow down from 50 approaching a roundabout, as you're rolling at about 10-15mph at the entry to the roundabout put your foot down.... A lot of DSG (and other auto boxes) have an uncomfortably long delay in circumstances like this - which is a situation in the real world you encounter a lot.
Otherwise read up on common issues and mechanical things to look out for, these go wrong fairly often and can cost a lot of money to put right. 2008 is starting to get on a bit in age so you want to make sure you're not going to be landed with any big bills