Very easy to convert and relatively cheaply too as many people ditch the kombi interiors when they convert to a camper so there's usually plenty of interiors going spare. The rear interior layouts haven't really changed from T5, 5.1 through to the T6 and T6.1, it's mainly been cabin and exterior changes.
The van and kombi shells are different but depends if you want to put the effort in. Vans dont have seat belt anchorages or floor reinforcement for the seat mounts and the side panels aren't bonded together so you'll have a gap around the window frames which you can trim over if you're clever.
There's quite a lot of hidden stuff too like all the hush pack and extra toys but if its just for short trips and you're not that bothered about the noise then its probably okay. Retrofitting an underbonnet hush pack would be almost impossible but you'd get a reasonable way towards the same level of effectiveness by adding insulation on the cab side of the bulkhead.
You should probably go blag a test in a kombi and then jump in a van (without a cargo bulkhead) and gauge the difference in noise insulation. Another option is to start with a Shuttle and take bits out to create a kombi. Shuttles look decent value at the moment.