I have used portable A/C's and they are a last resort in my opinion. They have a feature called a compressor delay. This means that if you hook your a/c up to a thermostat and the thermostat turns the a/c on, there will be a delay (about 5 minutes) before the a/c compressor actually kicks on. the problem is this creates a lot of temperature swing and burns a lot of energy because the portable a/c will just blow air (often warm-hot) for five minutes until the compressor kicks on to provide cold air.
If at all possible try using a window a/c in the attic and vent the cold air into the grow. I just put a drip pan with a drain tube under the a/c to collect condensation and run the drip tube to a drain, possibly your bath tub/shower.
Portable a/c's are inefficient and expensive. I thought I would give you some insight before you spend the money.
Its also january, can you just pull in cold air from an outside source? that would only cost you a fan and ducting along with a passive intake.