DSG clarification please.

Hello, I wondered if I could seek some thoughts or answers to a couple of specific questions in relation to the DSG gearbox on a Skoda Octavia. I have read alot on the gear box but for every horror story I can find someone who swears by them but cannot find the specific answers to the following questions and wondered if anyone on here with experience would be kind enough to let me know their thoughts or answers before i pick up and pay for the car.

1.)Can a seller mask faults in a DSG gear box?

(I test drove the car for 20 miles and messed about with the gear box and it behaved beautifully. So I took that to mean it was all working fine. I tried it both in sport and in drive and reverse. I nailed the throttle from rest and did an emergency stop and all was spot on. In normal mode it showed P R N D S on the maxidot screen and showed which one was selected, then moving it across to manual the P R N D S was replaced with 1 2 3 4 5 6 . I am guessing this was correct. Nothing was flashing.)

2.) How does the car tell you there is a fault with the DSG gear box?

3.) Is it definately the end of the world if the oil has not been changed at 40,000 miles. the car has a stamped service book but the skoda garage that serviced it has gone bust so cannot confirm that the oil has ever been changed. Car on 80,000 now so will get it changed now anyway. As I say it appears to behaving perfectly and some people seem to advise oil is not needing to be changed every 40,000 so dont bother.

Any thoughts or experience on this from you guys would help massivley beofre i pay for and collect the car. thanks Ben.