Not going to be cheap that's for sure, possibly even more expensive than going and buying one off the shelf as yours will be bespoke.
Order of build should be
1, all windows, and roof lights, openings for toilet cassette, water and power inlets, also for flue for heating equipment, then you need to look at 12v and 240v wiring, water pipes, gas pipes and waste pipes, believe me there is a lot of planning to do long before you make any holes in it.
2, Insulation, I'd advocate spray foam £300 + but it gets in everywhere, some tidying to do if you don't get the masking right, but it is the best, and also provide the very much needed vapour barrier.
4, fit water tanks and LPG
5, pre fit all equipment and test for leaks.
6, make/fit all furniture
7, refit all equipment
8, test for leaks.
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