If this shop is going to ANY heavier stuff, even if it's in the back of your mind, go with 6 inches minimum on the floor thickness. Another important thing is to remember is that if your slab shifts from a crack your tubing will be junk as it will be pinched or broke. I can NOT stress enough to go overkill on building it. To me, it's not overkill, it's avoiding a future problem.
Here is my install. This was about 8 years ago and had absolutely NO problems with it. I did all the excavation and construction on it and took care to not disrupt the base for future settling issues. 4 inches of pea gravel, insulation in rolls with vapor barrier built into it. The joints had tape on both sides. Around the edges I used thick styrafoam with 45 degree angles cut at the top edges so you can't tell it's there when finished. It has not cracked anywhere yet.
Personally I do not like road mesh as it's always on the bottom. I use 1/2" rebar 16 on center and wire tied together. This makes a great grid to attach you hose. I used simple plastic Zip ties with no problems. I found a company that makes poly rebar chairs and they are sweet. They make them in various sizes every 1/2" . They snap onto the rebar and hold it in the center, or where ever you want it as they have nice round flat bases.
I would also recommend doing a pressure test BEFORE pouring cement and keeping some pressure on it while pouring. have splices ready in case of a boo boo and if you hit it. That way the air bubbles will show. This is another reason I do NOT like mesh as you have to stick an object into the concrete to pull up the mesh. Not worth the risk to me.
Some of the pics are picture of picture so the quality isn't very good, but the best I cab do at the moment.
Also be REAL careful with your drainage. You don't want to have underground water problems so at least tile around the outside to give it a place to go.
I would NOT recommend a boiler in living space or some place you work. Opening the door at the wrong time will smoke you out and why take up shop space for wood storage? I built a building for the boiler and a years wood storage(13 cord). It's 20' X 20' with cathedral trusses so I can lift it out if need be with overhead boom. Keeps me AND the wood dry. Put in a big exhaust fan to get smoke out but if you watch your burn cycles it's not too bad. I laid concrete block around 3 sides and filled the cores with concrete, running a 1/2 rebar in every other core. I can stack the wood up against the walls with no worries. No termite problems with this either.
On the open side shown above I stick framed sections and used screws to put it all together. Would take a couple hours to get it open and the walk in door stays. Nailed 2x4's on the outside to screw metal siding on to match the other buildings.
Here is a finished pic.
Another thing to consider is to make the shop a separate zone using a heat exchanger. Reason being, that if for some reason you don't want to heat it all the time, you can run antifreeze in that zone only. Your tubing won't freeze and burst with the antifreeze if you opt for not heating it. Separate zone will save you a ton as the antifreeze is not cheap. I run mine all winter long so it's not an issue for me but something to think about.
Here is my plumbing layout. Water heater is out of the pic on the right.
There's a lot of things to think about when doing this. If you are planning to stay at this residence for a while, do it right.