For me, the things that are essential for a conversion are:
- Compressor fridge - not a cool box. It'll cost you north of £400 but worth every penny as your leisure battery will last a good few days between charges with it on all the time. And you'll also have a freezer. Gin & Tonic anyone? With solar you can run it indefinitely.
- Leisure battery and split charging system (i.e. Willington kit etc). Essential really.
- Solar panel - I don't ever use hookups (or campsites) so it means I don't have to worry about power. Small outlay for infinite power and you don't accidentally knacker your leisure battery by flattening it (the solar charger can be wired to cut off the battery at low power)
- Low coolant alarm.
- Make the bed wide as you can - as little as 8-10cm wider than "standard" make all the difference for two people.
- 12 volt sockets - no need to go mad. I personally never use my mains sockets as I never hookup. Plenty do though.
- Get a battery charger / PSU built in so when you hookup not only do you power all your gear, but your battery gets topped up. Nice to do the night before you go away to get the fridge down to temperature and give the battery a healthy top up. With solar this isn't really necessary though.
Everything else is personal preference. For me, the kitchen is essential. Sure, I cook bacon sarnies outside frequently, but only when the weather is nice. Every morning my g/f makes tea whilst I'm waking up, and she sure as hell won't go outside to to that when she's nekkid

Also when I'm at car sprints/hill climbs she cooks and makes tea in the comfort of the van. This is England, it's often cold and damp.
p.s. There are insurers who will insure it as a camper without it saying so on the V5.