My 2p…
Look for a tidy T5 facelift model with aircon and electric window as a base.Look to spend about £11-12k inc Vat. Engine (hp) is less of an issue as the only difference between the lower powered models and the 140bhp ones is the 6 speed gearbox. A decent pendle remap will get you a reliable 140ish Bhp for sensible money.
Then look at getting some caravelle rails and caravelle folding rear seat fitted. This won’t be cheap but it give brilliant flexibility and a bed if you ever need one. The fitting of the rails is probably best left to a specialist as you need to drop the fuel tank.
You can ply line and carpet and insulate it to a nice standard for very little money and it’s easy DIY (I’ve just done this with a mate)
Side windows aren’t overly expensive and can be fitted by any competent window fitter.
Spending wisely and getting stuck in you could just about scrape inside your £15k budget
Lowering is next on my list
Have a look round the T5 forum for any number of specialists, or let me know and I’ll pass on any info I’ve picked up in my year of T5 ownership.
A few before and after pictures of mine
My Van