Second, tackle the drainage problem, that has to be done first and correctly if not it will be a headache and could devestate your dream.
Thirdly, with inclemate weather approaching, do the outdoor stuff first so you can work inside during the winter. Don't know where you live.
This is all my opinion and others will and should have new ideas..OK
After the drainage is fixed then I would foundation up the garage entrance with the same cinder blocks again. go all around the garage and make the land fall away from the garage on all sides. The garage should look like its sitting on a hill. Install gutters and adequate downspouts for water to go AWAY from the foundation.
Build your floor and I would use decking of PT wood. Build the floor up with deck blocks and treat the dirt below with Centricon Termite Centers.
Determine the windows and door areas, determine the electrical needs, the insullation requirements, the access to the attic, like a disappearing stair case.
Install allot of outlets and have them higher than usual. tear out the old wiring and use new stuff.
This should get you going.
First, your money and time are wasted unless you get the lay of the land addressed. you do not want a swimming pool under that deck floor.