I looked at converting, and have helped my son convert a transit before.
Second hand base vans are a nightmare - so many awful ones, thrashed, fake MOT's and history etc.
On the Ford be careful of rust (I said be careful of RUST) and some of the dodgy ecoboom engines which blow up. Did I mention rust? If not, check for RUST.
I bought a ready made, from a big, very well established company with a reputation for being 'The Best' and are the oldest VW convertor in the UK.
It was only £1k more than a lot of shonky conversions or £5k more than me doing it all myself. But it is so much better thought out, better build quality, was a camper from factory so has proper suspension for the extra weight, insurance is cheaper and easier, the builder is brilliant at supporting my vehicle even though it is 10 years old - one email or call and they can post a part of share the spec so that I can fix it. It is a delight to use, and the only criticisms are things like one of the curtains does not quit close as neatly as I would like so lets some sun in, the seatbelts needed a small elastic to keep them from rattling while driving etc.
Mine was one owner, low miles, undersealed and doted on for the first 9 years of its life. I bought it with a full year of MOT, new tyres, new brakes, new batteries (van and leisure), new suspension bushings all round, timing belt replaced, fresh underseal with 10 year guarantee and a habitation service....
I bought it nearly a year ago and have been offered 10% more than I paid already.
I really really will not touch a conversion without all this back up again.
Look up Bilbo's campers.