Hey dillest, I have some answers for you. You may or may not like them depending on your situation. In terms of air conditioning for a sealed room, you have three main options. From best to worst, mini split, window unit ac, and dual hose portable ac.
The mini split separates the condenser and removes all heat from the room very efficiently. There is no air exhaust since the only thing in the room is the cooling coils. It's the most efficient and quietest form of cooling, but also the most expensive. As a rule of thumb you want 3000 btu's of cooling per 1000 light. Buying and installing a 12000 btu mini split will set you back at least 1500. And this is a job you can NOT do yourself. You need a professional hvac guy.
The best alternative is a window unit. The condenser uses outside air to cool the machine parts and inside stays where it should, inside. There is minimal leaking, mostly around the window itself. It's efficient and effective at the job. The main problem is one, if you're in the basement you probably have few windows and even if you do this will look suspicious. Also, if you're in a cold climate they tend to freeze in winter.
And now I bring you to the cheapest, easiest, and least efficient method of cooling. One I finally decided on for myself. There are two must have amenities if you want a portable air conditioner. First and most importantly, it must be a dual hose system. Secondly, it's very helpful to have a system that exhausts water vapor instead of emptying a bucket of condensation every day. The dual hose system sucks air in to cool the machine, and exhausts it back out with the second hose in a closed loop. There is little to no air transfer, but you do still need a place to exhaust. As long as you seal that well, you're good. It's not very efficient, and quite expensive in electric, but it does the job.
Hope this helps!
