Do it, you won't look back. Would never be without ours
Yes, VWs are expensive, but hold their value incredibly well. We bought our T4 Westy for £11.5K two years ago and I could sell it for the same tomorrow (not that I'm going to mind).
Yes, you could buy or DIY a a builders van conversion, but it won't be so well kitted out and you may have problems with insurance.
It makes a HUGE difference what time in the year you buy… you will pay top dollar now, but wait until the Autumn and you will get a much better deal.
If you're not looking to hold onto the van long term, and can't be fussed with the hassle of selling when you get back, you could also consider buying from somewhere with a guaranteed buyback scheme – lots of dealers do it. Cheaper than hiring long term and a good escape route if you find you don't get on with it, but personally I would just buy something decent and have faith you'll be able to sell it on next season.