Welcome,
When it comes to what to look out for there's not to many known faults.
In general just make sure the air con works properly, that's its been serviced on time (cam belt should be done at 4 years or 60,000 miles), the DSG changes should be smooth so look out for clunky changes.
The DSG boxes can go wrong but I think this is only a small percentage. Do a search on here and you'll find plenty of info.
As for mapping a lot of the users on here have their car mapped without issues.
Look at some of the reputable tuners and I'm sure you'll be fine. Just look at other people's cars on here to see what the majority are running.