Hi Spooky. Only Performance models need a diff service, the standard GTI doesn't. Other than what you've mentioned, they are pretty robust. Just the usual checks you'd make with any secondhand car. Beware the diamond cut alloys though, they can only be refinished a few times so if they need a refurb but have been done previously, that might be an issue. They can have a fair few internal rattles which can be irritating but many of them are common and easily sorted.
What spec are you hoping for?