I've done the same conversion on my TDI Ben, it's quite simple as Jay said. It's defo a 2 man job! But offer up the gti bumper and mark up around the radiator area then using a dremmel or similar trim it to fit BUT BE AWARE, do this a small bit at a time! You can always cut more, if u cut too much u can't add it back!!!! Once the centre is cut and all fits well that's it!!!!!! Just remember that you can use all the arch screws for your new bumper but u may need to save the spring clips (or whatever they are) from your old bumper.
Here is the eBay item number 111032725859 for the headlight mouldings I used, I then got them sprayed in my colour. They are not geniune but a good fit on my bumper. MAKE SURE THEY ARE A GOOD FIT BEFORE YOU GET THEM SPRAYED! (Cost me £35 to get painted)
This eBay item number is for the fog grills that will be fine for what you need 111173031787 again the same as what I used and I think it's the same seller as the ^ mouldings! These just click in place and I used a self tapping screw to keep them secure (there is a hole there to screw it into)
Fog lights, now this could present a problem!! Here are some that will be suitable for you eBay 171213853484 BUT, if you car doesn't already have fog lights then retro fitting them to work off the headlight switch is not easy! You have 4 options (some1 will correct me if I'm wrong!)
1- just leave then disconnected, they look good still but MOT time you will have to remove them as they should work if fitted!
2- wire them up to a stand alone switch that's hidden ( not very OEM)
3- wire them up with a relay to the main switch (some guides on the web) this gets around the CCM and coding but u need to be good with electronicals!
4- buy the correct module, wire loom and get them coded!
Hope I've been some help to you!!!!