I'm not sure there is any advantage to doing a garage in stages. I suspect you would be better to put the cash away in a savings account until you have enough to do the whole thing.

I suppose that, if you need some hard standing, it might make sense.

Plans can be drawn by anyone but you need to know what the rules are for your area - here, for instance, the roof-pitch had to match the house. Also, depending on the size you might need one or more reinforcing piers in each wall - they can be a bugger if there isn't enough space between them for your bench/toolbox/compressor/whatever, or you keep bashing the door against them.