I'd think you could build anything one stage at a time, but I think it'll cost more to do it one stage at a time vs. doing it all at once. Most of the garages I looked at were kind of "pre-fab" if you will. So, not sure how much you'd save by just doing roof and then adding sides later. The big cost for mine was land work, concrete, steel/roof structure fabrication. I'd think the siding would be the cheapest part.
If you are trying to minimize initial outlay I would:
Put the siding on and an inexpensive sliding door.
Wait on the concrete.
Does it need to be 20' high?
Mine is 16' with a 14' high door.
Just putting the poles in with the roof on would concern me:
It needs diagonal bracing. The cost of that bracing could pay for the wall purlins. Then you only have the cost of the metal on the sides to weather-proof it. The siding adds the much needed strength to the walls and protects the roof from being blown off.
Also , I rented space in industrial buildings/warehouse to store my coach before I built my shop. In some areas there is unused space. Insurance requires that they keep the sprinkler system on and keep it above freezing. It was much cheaper than renting an unheated storage unit. I realize heat may not be a concern in Alabama!!!!
Here in Indiana I found a truck repair shop that was leasing unused industrial building. He had more room than he needed so I and a buddy stored our coaches there. I liked the idea that there was someone there everyday. Also I had them do some of the maintenance on my Newell while it was there.
We built our bus bay 20' by 60' with 14 x14 doors on each end. Also included a sewer dump and 50 amp service. I think the bay is just right. Gives you room to work on each end and sides. The bay was poured at 6 inches thick with rebar reinforcement. In the center is a floor drain which I find very functional. Our unit does not have slides but would accommodate them. Hope this helps
Bob and Gerri
As bob says, a drain is very nice to have once the garage is enclosed. if it fits in your budget.