They're not going to "void the warranty". That's not even a real thing. If your tinkering caused an issue that would normally be covered under the warranty and the dealer can show that you caused the problem they will not have to repair it under warranty.I'm in similar boat, though haven't bought the Transit yet. Installing the Rostro CC unit involves cutting wires, and I can just see Ford voiding the warranty in that case. Cruisecontrolstore (Rostro) does claim to have a Transit unit now.
If you add a set of lights that isn't going to affect the exhaust warranty or engine warranty. If you change the seat covers it's not going to affect the tire warranty or electrical systems warranty. (unless you destroyed the seat airbag system while doing it)
If you clip wires and the computer burns up you will have a problem (if they discover it) but the rest of the warranty still applies to the rest of the the vehicle.
Adding cruise control is pretty easy to do. Most vehicles have the connectors already in them near the throttle cable housing. Generic systems use either an abs pulse sensor or a driveshaft pickup. I used one of those for over 100k miles and never had a problem on an astro cargo van.