How big does it need to be?
You could do a lot worse than the Mazda Bongo. They have proper, factory installed seats. If you'd seen how many of the other retrospectively fitted seats were installed, you'd avoid them!
I used to convert them many years ago. Brilliant vehicles. No idea what prices are like now but I'd imagine they'd be well south of £20K.
Here are my old ones;
http://www.midosconversions.co.uk/5.html