We put it in our house over 20 years ago. Put it in the concrete about 1/3 slab thickness from the bottom. A standard 40 gallon, 40,000 BTU/hour gas water heater heats the house and provides all of our domestic hot water. We have 3800 square foot of concrete slab weighting 72 tons. The key is insulation. Peak heating demand at zero exterior and 70F interior is 23,000 BTU/hour in cloudy windless conditions. Of course 0F occurs rarely. Here in south central Missouri, the system idles. The cost is very low since the house is super well insulated and the heating load is super low. My system is very homemade. No fancy headers, valves, or controls. It has one small 80 watt circulator. For example the headers are simple soldered together copper pipe with tubing connectors. The tubes are held by regular hose clamps to the connectors. There are no balancing valves etc.
There is an great amount of free design help on Build-it-Solar: http://www.builditsolar.com/
For a garage, you most definitely can do it yourself. It is very forgiving of mistakes. Much more important is careful design and construction of the floor. Good drainage of the sub-grade. Careful compaction of the sub-grade. Vapor barrier and insulation well done. Tubing in accordance with your plan. Pea gravel concrete from the local ready-mix. Pressurize the system during concrete placement. That will keep the flatwork guy from cutting the tubes because he knows the gage will go to zero if he does. The concrete guy who did mine learned well and now puts in a floor or two every year.