Being a builder with all the tools and knowledge I bet I could do it for that. 2nd hand parts and having expert carpentry skills. I converted my old T5 with all the electrics, sound deadening, windows, rear 3 seater bench, leisure battery, inventor, carpet etc for under £2k. I can buy online the template kit for interior layouts for £100. Buy the lightweight ply and make up units. I used to do upholstery too so can make a seating area.
Don’t get me wrong it may not look like some do it but I’d rather spend the extra bucks saving for my bucket list visit to Bora Bora!
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Oh and I know a Polish friend who supply’s and fits pop tops cheap so this all falls into my £12k budget.