I've done a dyi oil change on my GTI last weekend. It was a childs play, really easy to do. You can get a Mann filter (HU5006z) which is the OEM supplier for VW but cheaper than the one sold at the dealership. You can find it easy at any car parts store. Even Eurocarparts or Halfrauds stock it. Sump plug can be reused if you take the existing one out carefully as to not damage it. They are fairly cheap anyway, I have replaced mine as I had 2 spare anyway. Oil you can use any VW 504/507 spec. I used Castrol Edge 5w30 LL, the GTI takes exactly 5.7 Litres.