There are many guys on here who are more knowledgeable than I, but not sure how often they come on here.
It sounds like you still have a fair amount of research ahead of you still, 'cause some of your questions are not necessarily Haldex related.
1) The Haldex coupling is normally disengaged until its controller has decided it needs to engage. The controller is mounted on the Haldex. It monitors wheel speeds, steering angle etc. via the canbus, and engages when it sees fit. If you want to keep this self regulating function, you will need to fit ABS electronics from a quattro car, and ideally all the sensors related to stability control (yaw, steering angle). You can skip these sensors and disable them with VAG-com if you know what you're doing.
Otherwise you can run a standalone Haldex controller such as the one offered by F-town streetmachines.
As for the angle drive, you cannot simply bolt one onto a FWD tranny. Ideally you would run a 4 cyl. 02M quattro box. You can get them in 5 or 6 speed. Installing one requires some work, but it is a definite upgrade from the 020, 02A or 02C.
Hydraulic clutch is the same as for any, just need to cut a hole in the firewall and use a reinforcement bracket, then bolt on a hydraulic master. Then you
Just need the correct line to hookup to whichever style slave you end up with.
Speedo is standard electronic VW. MK3 should work fine.
Hope that helps!