Alright mate... welcome to the forum.
Here are MY opinions on your options (other opinions will follow and not necessarily be the same

)
5 Door GTi MK6 (poss 3 door for coolness but 5 door holds value better.)3 is cooler, 5 is more practical. I've never heard of 5 holding its value better - but this may be the case?
If you have passengers, get 5 - otherwise get 3 if you're never going to use them!
1.Colour = Carbon Grey (white and red to showy, got black now)Carbon Grey is one of the best colours around. If I was having a GTI I'd probably take it!
2.RCD510 + Dyn sound packGood choice. Dynaudio is excellent - and if you don't need the SatNav, then the RCD510 is a great stereo. I would take the reverse cam too - for an extra £150 it seems rude not to! Maybe thats just me?
3.CruiseUseful if you're doing a lot of miles. Bear in mind that most dealers will fit cruise for about £150 which is about £100 cheaper than the factory option.
4.ACC I took this because I'm scared that I'll regret not having it. There is a noticable difference between the settings and the guys who actually have it rate it highly.
5.Storage Pack.I have no need for this - but I'd be dubious of it as it restricts the foot room in the rear seats.
6.Leather or Cloth If I'd ordered a GTI I'd take the cloth and the Winter Pack. The only reason I took leather is because the R cloth seats have white bits on them which will get ruined quickly! £1730 for seats is a total rip off... and the cloth seats in the GTI look great.
7.Phone PrepI have ordered an aftermarket Fiscon kit - which is available for £300 in the group buy section. It means fitting it yourself (or going to one of their fitting days), but it offers much more compatibility than the factory kit.
8.Ipod ConnectionYou'll get a USB connector in the armrest that you can connect a USB stick into or your iPod. Both will display on your RCD510 and allow you to navigate through them very well.
9. NO to DSGI took it as I regretted not having it on my ED30. I loved the test drives and thought I'd give it a go. I may love it, I may hate it... time will tell!
10. Can you swtich the ESP of on the MK6Hold the ESP button for about 5 seconds and it will go off. When you restart the car, it will come back on. Handy to be able to turn it off for snow!
11. Having the 18" Monza'sI'm not a fan of Monza's and never really liked them on the MK5 either. The majority of owners love them. 18" are a must for Monzas - or eBay them and buy a nice set of BBS 19s!
12. Would love Xenon's but its that or the leather really.Given the choice I would take xenons. They are a must for me. A very expensive option, but well worth it!
What do you think? Also, any one know the difference between the old engine and the MK6 version.There are a couple of very detailed threads on this if you search!
Once again... add the reverse cam! Its cheaper than parking sensors and a quality gimmik to show your mates!