Hi,

Units / beds come nearer the end of a conversion although it is really up to you. Here's what I'd do.

1) Sit down and work out everything you want in the van and all the tasks.  That will help you work out what's what.  Some will obviously have to take place before others e.g if you plan to treating the metal floor of your van to seal against rust this has to be before you lay carpet.

2) Windows come near the start and are best put in pretty much when you have an empty van shell.  Any cutting of the metal body of the van is best done at the start e.g. for windows or a roof vent.  The main reason is it is a messy task and creates loads of metal filings which you want to be able to clear up (they rust and you don't want them stuck down behind your lovely units).  This also gives you the start of some parameters on where you put other things e.g. you will probably want to fit units closely below and to the side of the window.

3) Before putting in units you want to have laid all your electrics, cabling and any plumbing - water hose, gas pipe etc as these are things you want tucked away (although remember to leave access e.g. to your leisure battery or gas bottle).  Think about what is hidden in a house and you want to put this in, then do your insulation, panelling, units etc.  It's a bit chicken an egg as you need to have thought about where your water tank and stove will go and in what units to work out where to lay your electrics.  Sketching it all out with a task list before doing anything is the way to go.

4) Generally furnishings, curtains, cushions, carpet etc should be done last just to keep them from getting messed up. 

5) Things like seat swivels and window insulation mats can be done whenever, although I'd fit the swivels when I was doing the windows in case it's messy and when there's not much in the van.  It's easier if you have access and are not trying to negotiate around units or a bed

Good luck.