OK, let's run thru them one at a time:

You should use a product like Weldon for gluing. Make SURE you test the sump before installing! Also, since you are building it yourself, make sure to bevel the exposed edges at the top slightly. It will save all those tiny little cuts on your wrists and forearms!

I prefer the drains to go all the way into the water. They will drag PLENTY of air with them - google "durso" and make sure you use them with your drains. It will quiet them significantly

If you put them into your garage, make SURE you have a good air exchange in the garage. If your chiller is trying to chill using 110 - 130 degree air from your garage in the summer, it will 1) fail and 2) shoot your electric bill thru the roof. While many chillers are designed to work in environments up to 100 degrees, most aren't, and if they do, they won't work well!

Otherwise, the garage idea is GREAT. Spills, etc. are much more manageable there.