I havn't read the linked thread, as tbh I cant be arsed
but you dont need to use the 1.8t box, and you dont need to convert to a hydro clutch
what you need to do is get an 02a box from a 16v gti and use the seat ibiza clutch cable setup, (the reaon they use a mk2 box in the qpeng guide is because there putting it into a mk2!) this will allow you to keep a cable clucth and save messing about with pedal boxes. you cannot however use the 8v gti box as it is too weak and will break
In regards to the qpeng route, its been covered many many times, but general census seems to be qpeng is easier but more expensive and some people dont think its worth the risk of running the engine without a knock sensor (again info on this can be found using the search function, or for general info just google 'knock sensor')
The plug and play harness is handy to have and saves a bit of work but the engine management qpeng uses is far inferior to running oe management.
I'll be doing my conversion for around £2000, but im doing all the work myself and have also managed to get some good deals (AUM ecu plus remap for £300!)
First thing you really need to decide is which engine you want to run, as there are a fair few different 1.8t's that are compatable. Im using an AUM from a mk4 gti which runs a ko3s turbo, and is a fairly straight forward swap. I know others on the forums have used other engines though so it would be good to hunt around for info on all the variants and see which is right for what you want