Lots of questions. You are probably going to need to French drain to keep the water out. You need to remove the large door they do not insulate very well and you will loose a lot of heat. You will need to check the box at the house and regulations in your area. You will probably need to conform to building codes if putting in power and will need a permit. Better to save up a few months and have that done. You need insulation and I recommend on a cement floor to use the insolating layer under the flooring and you will loose a ton of heat there. I would put a lower ceiling with some storage above but will still allow you to heat at a more nominal cost. Trust me I heat an area like this for my sewing room and it ain't cheap. We wired in in wall heaters when we put in our insulation and drywall.
I would have a contractor give you some bids so you have a realistic idea on how much this will cost and what the true work actually is. Also what permits will be required