Not difficult to fit them at all, just fiddly trying to hook up all the wiring. It should all generally be plug and play, at least that is how they are designed. The Autoleads stuff is more keenly priced. You should be able to get all the cables required for around the £50 mark. I am waiting on confirmation on the InCarTec prices as that harness does seem to be on the dear side.
Out of interest, what HU is in the car at the moment?