Ouch, that's a bit cruel oh wise one...
The Iveco is an absolutely brilliant van to convert, but prior to starting, from what I know of them, make absolutely certain that any and all of the dreaded tin eating monster worm (rust) has been well and truly treated and eradicated.
I agree 100% with Trev, rip out any and all of the non Iveco wiring loom wiring, but don't throw it away as some of it might be worth using during the build.
But far more important than anything else, is to make absolutely certain that before your build starts, you plan, plan and plan again even the most finite details of your build so that you get your build 100% right from day one.
Enjoy