I have a portable at two locations in small equip rooms. They work fine, and really help. It is definitely worth the $300. It will keep it at 80 or so on a hot day, plus dehumidify. Order a 25' aluminum duct hose (5 inch dia) from Grainger or McMaster-Carr. Cut hole in your ceiling close to the wall. Run the hose through the hole and lay it over your soffit vents; then connect to a/c. It is that simple. I leave my set to 75 all the time.
I have cental air in my other 3 equipment rooms. I would not build one without cental air today. But portables are ok in the meantime. Plus it beats cutting a whole in the wall for a window unit.
Get the biggest portable you can. Mine are 12,000 btu (1 ton) You only save a few bucks going to a 9 or 10,000 btu and is not worth it.