Probably the most crucial bit is how much can you do yourself as opposed to how much do you think you can do yourself.
Materials starting from bare shell, with everything new and at least reasonably high spec / quality, will cost anything from 20 - 25K. You can knock say 3/4k off that if it's spray foamed, battened, lined and wired with first fix and has a water tank.
Eight months full time to fit. If you take an average wage at 20K you 'pay' yourself @£14k. Professional hourly rate subtantially higher.
Therefore your costs are roughly shell 20k, materials, £20k, plus 'your' wage 14K.
You need plenty of skills, plumbing (water / gas / c.heating), electrics (12/24v + 240v), woodwork etc etc.
Its a big job and you'll probably need professional help at some stage.
No doubt someone will dispute these figures, probably upwards.
Bon chance.
Jo