Hi mate, welcome along in yet another diseasel!
Definitely go for a remap, 200bhp is easily achievable. Not sure that you will need to do the brakes, iirc the GT170 has the same set up as the GTi on the fronts anyway - 312mm discs. Xenons are an excellent upgrade, although to save a few pennies you could upgrade the OE halogen bulbs to something like Osram Nightbreakers or Ring Xenon Max, obviously no comparison to OE xenon, but a lot better than standard. The sat nav of choice is the RNS-510 or MFD3, especially the later version with LED touch screen. I have the MFD2 installed in the Golf, it's ok but I certainly wouldn't have paid the £1500 VW wanted as an optional extra.....
Some things you may want to do:
GTi rear valance (colour coded)
GTi front spoiler insert
GTi side skirts (colour coded)
LED interior, puddle and sidelights
Tbh the list is endless, and joining this forum is very bad for your bank account!!!!!