Sam - how about a build thread on here, I'm sure plenty stw'rs would be interested in more detail on that - budget solutions are always best!!
For OP, I spent a year in a $1,000 van touring australia. It was a lwb Mazda window van (similar to a modern Toyota hiace favoured by shopkeepers and florests). Fitted out with a full size futon fixed to batons along the sides allowing loads of space underneath for bags and boxes for camping which left about 4ft of space between the back of the front seats and the end of the bed. Never had proper fitted kitchen but normal camping stuff that could be used inside if raining but as oven as not cooked outside. Had a massive water barrel thing with a wee tap (circumnavigated Australia so took water pretty seriously)
stuff I would want if I was doing it again:-
swivel captains chairs - never had them in my van, would have made much better use of the space tho
longest van you can get - length is king, anything else is luxury
decent stereo - again I didn't have one, but vans are pretty noisey so decent sound are a must
fridge - only needs to keep your dairy and a couple of beers cold but a proper powered fridge would be great in Europe
power sockets to keep computer, phone etc charged
properlly thought out lighting to add comfort
roof vent so you don't have to keep windows open
and make sure your converstion is as stealth as possible - no alloy wheels or campervan graphics. If it looks like a builders van then you can sleep anywhere even large citys
I don't see the need for:
built in wardrobes - unless hanging your suit on a hanger is a traveling priority! Just pack bags and boxes sensibly and have them accessable.
Plumbed in sink - seriously, why??
Defenately do it - no need to spend megabucks, cheapper the better as you'll spend. Shi7load on fuel anyway!