Hi Mino, first if you want to save a few bucks, skip the mixing valve and make a mixing valve with three ball valves. I went whole hog with a big 1" inlet 1.25" outlet valve designed for OSHA drench showers. I found out I could have just used a $15 ball valve to accomplish the same thing.
I didn't use Pex, when I plumbed and poured this floor, pex was in it's infancy and VERY expensive. I had used on my last garage floor 3/4" black plastic irrigation pipe used by the sprinkler installers. If you keep the system under a slight pressure, and PUSH the fluid through the piping, the oxygen barrier is not a necessary item. The previous garage is going on 20 years, no corrosion in the system.
I do run some glycol with the water, this helps reduce corrosion too.
Keep the circuits equal as possible to reduce the flow differences. Shorter loops with small diameter tubing. I have 8 loops of 180-200' each.
Make some steel 4" diameter tubes with 1/4" thick plates welded on top. Torch cut an X in the center, this is a chain pocket. install several of thes pockets with the top plate flush with the concrete. You drop a length chain into the X, and then slide the chain into one leg, the cross link catches under the steel. This makes moving heavy objects easy by yourself, there is alway an anchor to attach a come-along to.
Another thing I did was to find nesting steel tube, I think I found 4" id and 3" id with 1/4" wall slid inside each other. Put several large tubes in the floor where you might want the ocasional vice, grinder, anvil all on a pedistal fo the smaller pipe nesting in the steel pocket in the floor. I use a vice for a project then pull it out of the floor and put the vixe and pedistal in a corner, out of the way till I need it again. I have an anvil, a big grinder, and a portable steel welding platform on pedistals.
Take photos of the floor layout, and keep them. I used mine to find the hoist anchor areas, then a laser thermometer to trace the hot water pipes prior to drilling the anchor bolts.
That's all I can think of right now.
Oh, You can use white steel barn siding for the ceiling, blow in insulation above instead of the sheets of foil faced foam. The white steel siding will reflect the radiant heat back down almost as well as the aluminum surface does.