Approx cost of turning a detached garage into dry storage

We have a detached garage in the garden which is approx 20ft X 12 ft which I would like to turn into storage for perishable items. My partner is / was a joiner so could do the work himself but due to time constraints it would take a while so I looking for rough quotes and timescale for the work to be carried out. It's not an immediate thing as it needs to be emptied, so I'd say before the winter! The garage is single skin brickwork, with a upvc window and tiled roof so will need DP membrane etc, insulation, battoning, plasterboard and skimmed walls., vents etc. We will be replacing the up and over door with French doors which we will supply but will need fitting. We will also need another light and plug socket towards the back of the garage. The wiring etc was replaced less than 5yrs ago and the garage in on its own circuit. The roof has quite a few broken, slipped tiles that need replacing. I'd probably allow for half of the tiles replacing to be on the safe side but it doesn't need a full roof and it's not letting any water in. I'm looking for a rough idea of cost and timescale to complete. There are no access issues and there's side access up to the garage as its a semi detached property. Thanks