I spent a year or so fitting my shed like a boat. I have a full 6' 6" bunk, a wood burner, various lockers and other stuff (fallen asleep in it several times). The floor is reclaimed 6" oak and all the cupboards and bits are charity shop. I've fitted a 240 volt ring main, so I've enough light to do my wood engraving in it.
My shed is a standard off-the-shelf version; pine interlocking boards, plastic windows, stirling board, etc. I added tongue and grove to the ceiling (like the deck-head on an old boat), a reclaimed oak floor, miniature skirting boards, a water capture system, Cuprinol (true!) dark oak preserver, an outside deck and real glass. I made the log bin, the wood-burner base from aluminium and slate. It took two weeks to figure out an insulation system to stop the wood- burner setting fire to the timber roof.