Why a 2 stall barn if you plan on having 3 horses? Now I am not a fan of stalls and mine stay out 24/7/365 but having lived in the south, the bugs can be brutal and a dark stall is the only place they can get away from them and maintain their sanity. In the heavy rainy season, you will want to restrict their access to the pastures to protect it as well. Their hooves will cut up a wet field in a few days and it can take years to come back. You also need to budget a "sacrifice" area typically attached to the barn where you can lock them up when necessary. You are not going to get away from providing hay. Might only be 30% depending upon how much land you dedicate to them, how it is managed, the variety of grass and the stocking rate. Budget at least 1 ton per horse per year. 2 ton would be more realistic. Grass grown on sandy soil will require tons more water than other soils. It is also easier to be ripped out of the ground and requires more time to get established because you need more roots to hold it in the ground. If you are planting traditional southern grasses like bermuda, budget lots of extra money for fertilizer. You will get decent yields but it requires over 200# N (that's available N) per year. You can and should amend the soil with any manure you generate but that only will contribute 20# N per ton (dry).
I would take a very conservative approach. You think the land can support much more than it can. If you think the land can produce X, budget that it can only produce 1/2 X. Your neighbors and extension office are your best source of relevant information for your specific situation. If you overestimate what you can do with the land and don't plan accordingly, you can spend years and lots of extra $$ correcting the damage that was done in one short season.