As said series 1 loom will make it the easiest swap, but if your going to do that it is best to run the series 1 ECU's, the engine makes no difference at all this way round, but if say you put in a series 2 loom you would be better off running series 2 ecu's, but the series 2 engine has a couple of extra sensors which are quite important to have in order to get the most out of it. All combinations of mixing will work and operate, just not as well as keeping things together.

My MK3 cabriolet was turbo'd by myself a year ago