Insulation of the building will play a fairly big role, you need to know how much BTU you actually need. What will you heat the water with?
I did my own system last year for the new house/shop. Its a very well insulated building(ICF), our winters aren't too bad at all here. Some -10C, rarely much lower than that.
I searched a lot, had 2 companies do an estimate which was completely NUTS and they wouldn't have done half the job I wanted for it and started to argue about what I WANTED. Then finally went to a plumbing place that supplies to regular plumbers and happen to also supply all the infloor heating stuff but don't install. They just sell at a regular FAIR price, and for free did their own estimate of what they figure would be needed. I compared that with my notes, made my own shopping lists, some changes.
I decided against running it for the 2nd level, just pre-wired upstairs for electric baseboards but didn't install them(reason to follow)
I used 2" foam under the slab, High Load 40 for the "garage" side. I have 10M rebar tied 12"x12", its on 3" chairs. Pex is 1/2" and in 250ft rolls, oxygen barrier, forget which brand now but it was nice stuff. I tied it aprox 12" in between, I kept a closer loop for the perimeter of the building about 4" from the inside wall. It took 750ft(3rolls) to do my 18x34 garage. I had about 7" of concrete poured which put the tubing about half way in the slab, along with the rebar. You can go with larger PEX if you want, it'll have a higher BTU loss per linear foot, and can also carry hot water a bit further if needed. Or you can run the 1/2" closer together, up to you.
Similar deal on the house side, but regular 30psi foam, and 4" concrete, 1" chairs, 15" between rebar.
I use a 12KW Argo boiler to heat the water. I set it to a max of 110F water temp, it will get to that if running one slab, but in the little testing of it I did it hangs about 90F if I try to heat both slabs from cold start. Its perfectly fine that way, no reason to run 140F. I do think putting a 16KW boiler wouldn't have hurt and I'd go with that if doing it again. They're only another $100 and like the 12KW, shut elements off when not needed anyway(meter just spins faster haha). But the Wire(copper) to power it is pricey(but you can size the wise to the actual amp of the unit, not to the breaker, according to our codes anyway)
But guess what.... My infloor heating hasn't turned on at all so far this winter, and never will. It will only be used if we go away for more than a few days, so it kinda sucks to know its just sitting there(no glycol here, straight water).
Reason for it not being used is I have one wood stove on the house side, running it a couple hours on soft wood gets the whole house(1600sq/ft living space) at 80F(or more). I have no heat in the garage other than what makes it through my office door, its usually between 68-72F in there(machines kinda warm the place up too).
You'll find that most " Professionals " will say to just throw a wire mesh at the bottom on the foam and many just staple the tubing to the foam,(there's large staples and a type of gun made for it) I think its a F-ing joke, sure its quick, easy, and they charge a mad price for it, imagine what they'd charge to do it right. Mesh is worth nothing in concrete, more so at the bottom(either way its too weak), and tubing at the bottom means longer response time, and it just plain makes little sense at all unless you pour a real thin slab(so do only pour 2-3"). Keep it closer to the top, mid slab made sense to me if I have to ever drill into the concrete a bit I know I have at least 3" of concrete on them.
Pressure test, and keep the pressure during the pour. I only had 30PSI, I was lucky the guys didn't put any hole in it(rough bunch) should have had 70-100PSI, keeps it from collapsing too much as they walk all over it, also quicker to find a leak if it pierces.
As for your actually boiler/manifold set up, that's a fun other list of things. There's many ways to do it, there's only a couple things I'd change on mine. So if you need info on that i don't mind take a few pics and doing more writing.
(post too long to bother re-reading, hopefully some of it makes sense...)
