Hi Natz,
It's great that you're taking a hard look at temp control before you get in too deep. Mid 80's is a bit too high ime for the best potency/trich production. If you can drop things into the mid to upper 70's you'll have great results. For my setup, a 9000btu A/C unit works pretty well.
I'm flowering in a 2 x 4 x 8 closet. If you put an A/C window unit in your room it will bring the temps down for you and your inline fan will pull enough cool air through a passive intake or the cracks in the door to keep things much cooler. If you can exhaust your hot air off the lights in the attic or outside a closet is totally do-able. I have a 1000w Quantum digital ballast with a Solar Max Dual Arc bulb for my main light. I also have a 6-lamp, 4 foot T5 fluorescent fixture mounted on the back wall for a side fill light source...
For temp control I have a window A/C unit in the bedroom where the closet is. My 1K bulb is in a Cool Tube reflector and I'm using 6" ventilation with a 630cfm Osterberg fan. For odor control I have a 40 lb. Can Filter mounted in my closet and attached to my cool tube. I route the output of the cool tube up through my closet into the attic where I run my Ostberg inline fan. The fan is inside a large cardboard box lined with sound absorption foam and the box is suspended by rubber straps from my roof joists to un-couple any vibrations and reduce noise transfer from the attic and I exhaust the scrubbed air into the attic.
As long as I keep the temps in the bedroom around 72, the closet stays around 76-78 degrees and the humidity stays around 45-55%, and that's even when I run my ballast at 1k with a Dual Arc which is a bit hotter than a HPS IME. The past week with high humidity and temps in the mid 90's it's been harder for my bedroom A/C to keep up. To keep temps comfortably in the mid 70's I dimmed my ballast back to 75% (750w) and it's been staying cooler and is plenty of light for that space.
I also put a post on a product called the Ice Box Heat Exchanger that may be worth looking at if you can't cut a hole in your closet to exhaust the heat. Here's a link to my current closet grow and there are a few pics of the setup.
http://www.marijuanapassion.com/forum/showthread.php?t=43035
Some growers due to heat shut down their bloom space setup for the summer and resume flowering in the fall when it's a lot easier to control temps. imho to keep a closet/tent grow cool A/C will have to be used probably half the year.
Good Luck!