Probably about the biggest tip I could give is to go round several dealers just to look at as many vans as you can. Set yourself a date before which you won't buy and stick to it. The idea is to see what layouts are available and which will suit you best. Try to imagine living with the van for a typical outing. Ask yourself where you'll store your gear, where will you dry wet clothes, are the beds acceptable, etc. etc. With a little luck, you'll find a range of layouts that will work for you -- and that's when you can start looking to buy.
Regarding older vehicles: habitation parts shouldn't be an issue as you can usually replace older parts with more modern equivalents. However, parts for the base vehicle (engine, transmission, suspension, etc.) are a different matter. That said, £15k should get you something under 20 years old and you should be OK if you choose a common platform like Fiat Ducato, Peugeot Boxer, Mercedes Sprinter, and VW Transporter.
Good luck.
