So does that mean you have a crawl space?
If so, I guess if it were me, I'd set a couple of cinder blocks on the ground and use some short molly posts and a header to just shore up the joists a bit under the tank... that is, if it's needed at all. Depending on where the tank is and how it sits on the floor joists, it may be unecessary. If the tank is sitting on a wall that is supported underneath, such as an exterior wall, and it sits perpendicular to the floor joists, you'll probably be okay without any reinforcement... though extra never hurts.
When I put in my 180g, mine ran parallel to the floor joists, so what I did is I doubled up the 2x10s (which was a pain because I had to cut a bunch of plumbing lines to do it), then I built a small wall running perpendicular to the tank directly underneath the center.... about a weekend's work... well worth it.
Yes, the flow through the sump will make a big difference. If it were just a tank, perhaps not, but when water is spilling over baffles or a skimmer is in operation - both of which dramatically increase the surface-to-air area of the water, it can make a dramatic difference. It all depends though... if your tank is 80 degrees and the garage is 70, it's probably not going to make a big difference, but if the garage gets down to 40 degrees, or up to 100 degrees, it might make a bigger impact.
The other thing you need to think about when locating a sump or anything else that exposes the water to air, is that in the garage, you potentially have fumes, dust, etc that can get into the water. In another if-it-were-me case, I'd probably build a small enclosure with a hinged lid or something that insulates and isolates the equipment to some degree. I will say that locating the equipment in a larger work area like you are doing has numerous advantages that makes a good case for doing it if you are so inclined to take it all on.