I would say just one (or two) things to throw into the pot. Donkeys years ago (about 1975) a friend of mine wanted to build himself a small yacht from timber, about 8 metres long with accommodation. He bought the paper plans. First he had to extend his garage at his 3 bed semi house. Having done that he set about building the yacht but being in his 20's he had to have a full time job as well. Trouble was, he had to use only the absolute best of everything. Marine grade stainless steel screws weren't good enough, he had to use special expensive silicon bronze screws which he could only get from Canada. Best teak for the hull and decking. And so on. He had two children so he made two small beds and a large double. Full galley of course. He could only progress the yacht as finances allowed. Children grew, so he had to modify the accommodation. Another child came so he had to modify it again. Finally he got round - or didn't - to painting. Only the best two pack paint would do but before he could get round to it the use by date came and went so the paint went in the bin. Then he lost his job and had to relocate to the Isle of Lewis because that was the only place he could afford. He built a trailer to tow the yacht and had it galvanised. He took the not-finished yacht and carried on rubbing down this little bit that had lost its varnish, and wondering where to get a lot of lead for the keel. He built a boat shed at his new house, installed oil fired central heating (in the boat house to keep the yacht from shrinking). Children meanwhile have grown up and left home.
About a year ago I got a text from his wife. Donald had fallen from a ladder, broke his neck and died. He never launched the yacht that he had taken over 30 years to build, because he always had to have unreachably expensive parts and he was far too much of a perfectionist.
My point is, you may be capable of doing a self build, but do it sufficiently quickly that you are still around when it comes to using it.
Secondly if you are thinking of resell value, DIY campervans are more difficult to sell, partly because buyers are wary of the quality, partly because you cannot re-classify it as a motorised caravan with the DVLA, partly it might be difficult to insure. Also there are far more rubbish conversions than good ones, and personally I have got fed up with scrolling through things with multicoloured flashing strips of LEDs, carpet on the roof and walls, rock and roll beds, and furniture which frankly looks like cut up pallets nailed together. There are a lot of youtubes where some american has cobbled a mobile office into a van, called it full time living and I think most of them are just click bait. They have no washing or toilet facilities, but they must have a fold down flap to use their computer on, a surf board on the roof and a mountain bike on the back. It discredits the whole ethos of going on holiday in a camper.
You might be able to resell it for more than the cost of the materials, but you can't include your labour at mimum labour rates. So reselling it for a 'profit' is a non starter so forget that. I was once a sort of 'rep' for a very high quality converter bloke - I took his vans to shows and sold them. He had a proper small factory workshop with all the right woodworking machinery and employing a couple of men. His fall down was that he regarded each customer as an individual and although he specialised in one brand of van he would always be making some modification for each new customer so he couldn't maintain a steady production line and he went bust.
Don't get me wrong, I'm not trying to put you off. If you can devote 6 days a week, 8 hours a day to it with 'unlimited' (by which I mean adequate - you can buy stuff whenever you need it) funds and skills and machinery, go for it and you will have something you can be proud of. It wouldn't be my choice, I think I know enough about motorhomes and the current crop of computerised vans to know that I couldn't do a job that would satisfy me so I personally will buy a ready made one. (A German one - I have had enough bad experiences of UK makes)
Only in my case, if I buy another one it will have to be on a Mercedes automatic base vehicle, and you can't find those in the used van market unless they have 350,000 miles at 90mph or coughing and spluttering round town grocery deliveries with clogged up engines.
........if it wasn't raining today I would be going to see a converter of BIG Mercedes and VW vans, in Corby. I like the look of his workmanship on youtube.