Welcome,
The
DSG is indeed a marmite thing. I personally love it. Like yourself I got it primarily because of the novelty but I would find it very hard to go back to a manual now. You do lose the clutch pedal but I think there are definitely more gains. In D mode my car is very smooth (perfect for taking people who appreciate a comfortable journey) and in manual mode I can drive how I choose - I never really use Sport mode as manual is a bit better I think. I also love the DSG pop it makes when you change gear whilst having a bit of fun.
Candy White is a great colour. Mine is red but I think the GTI looks great in white (and you also save a decent bit of money by not going metallic).
18 inch Monza Shadows are also a good shout. I didn't get these and are perhaps one of the regrets I have. Although they are vulnerable to kerb damage.
ACC is a further sensible choice. I know plenty of members on here have it (I don't although would maybe get it next time).
As for the leather... it really is personal preference. I prefer the look of the GTI tartan. It is after all unique to the GTI and I also prefer cloth seats for both the comfort and the durability. EDIT I find leather unsightly when it gets creased and marked.
I wouldn't say it is too loud. Orange seats (Focus ST)
are too loud in my opinion but I think the tartan (especially in the mk6) adds a nice class to the cabin. There are some images floating around on google and I'm sure there may be some pics on here. If your'e still undecided have a look at one in the dealer if you can.
Apart from that great choice on the car. It really is a brilliant car to live with. it's not faultless (everything has one or two niggles) but it does everything that needs to be done without fuss. The standard car is plenty quick and fun as it is (and if you ever want more then there's always a remap

)
Furthermore, some of your options, such as the RCD510 upgrade can be retrofitted (either by the garage, yourself or some folks on this forum) so be aware that you could save some money on upgrades such as this. I'm sure someone will be along shortly who can advise you further on this.
Well, that's it from me, and once again welcome to the forum.