It should be straight forward but very fiddly! You'll have to have the dash out unless you're sure all the wires in it are OK. Hardest part will be finding a donor vehicle and removing it all, what year is yours? If its an early spec (hazard switch on dash) you might want to consider converting to the late spec (hazard on stalks) electrics whilst you're there which are compatible with later dubs like the MK3/Corrado.
Replacing the fusebox is dead easy, just a case of pulling it forward and pulling all the plugs out then refitting in place, the hard part is getting all the wires out that pass thru the bulkhead