Get the best base van you can - preferably from a local classifieds off a local trader or individual (that way, you'll be better placed to avoid any undue 'scene tax' and/or vat).
Look for service history and how well it's been generally cared for. Think about caravelles to save on the expense of fitting windows to a panel van - they're also nomally better trimmed internally (plus you can possibly sell on the seats, trim and fittings you don't need.....). A compromise would be a window van
Depending on what you want inside will determine your conversion budget - but some REALLY rough numbers :
Pop-top roof - £2k
Seat/Bed with seat belts - £700-£2k
'Camping' interior (cooker, fridge, storage, etc) - £1k
Quite often, the 'little bits' end up eating a lot of the budget, but if you're prepared to DIY some of it, you'll save a fortune!!
For £8k, you might just stumble of a REALLY nice, already completed van - the choice really is endless, but a large portion of it does revolve around being in the right place at the right time.........
T: