converting panel van to crew van info needed please.

Hey folks this is my first year as a self employed gardner/landscaper. I have a 1999 berlingo which is big enough for transporting tools and equipment, and a family car. I'd like one vehicle that can do both, ie. a crew van. Problem is they're a lot more expensive than the panel van equivalent. I'm fairly mechanically minded and have restored a few classic cars and bikes as well as having worked in the commercial vehicle glazing business for a few years. I think I have enough skills to convert a panel van to a crew van but am wondering if there is much legal stuff involved. I know it would require a vosa inspection which is fine by me as this van will be carrying my wife and two year old daughter, so I want it to be safe.
Being my first year in business, I need to keep some capital for unforeseen expenses, so if I can save money building a crew van rather than buying one. At the moment a standard three seat panel van is enough, so I don't need to convert the van right away. However my wife is pregnant, so more seats will be needed later. This should give me time to gather up the relevant parts from commercial scrapyards, windows, seats etc.
I'm thinking of using a swb low roof transit or traffic as the base vehicle. Does anyone know if the bulkheads on either vehicle can be modified to fit behind the side door rather than the front seats? are there bolt holes in the floor for rear seats? Basically has anyone done anything like this and how did it work out? I know there are firms who will do the conversion for you but I doubt they would give up the information for free.

thanks in advance. John