Hi, Anne. "6a/6b" covers a lot of ground, and whether the expense of a mini-split system is warranted depends a great deal on where you are geographically.
Frankly, I doubt a mini-split system is appropriate.
You can use
this calculator to estimate the total heat need. A larger heater would probably be a better investment.
As far as cooling goes, again, depending upon where you live, the proper combination of shade cloth and a swamp cooler or wet wall may be a better choice, both for cost as well as adding humidity, rather than removing it.