Congrats on finding a nice ed30, exciting times.
Why not voice your concerns to the seller, perhaps he could offer a gearbox specific warranty or maybe change the gearbox oil for you if you decide to buy, he wants to make the sale after all.
I love my dsg, it will let you drive in a relaxed manner in traffic in d mode and will offer thrills in s mode for those times when your devil horns come out. Add to that this gearbox can be mapped!....amazing twin-clutch technology.
Have you seen Jason Plato's review on youtube? He loved the golf's dsg.
Good luck...