A few thoughts from someone who has toyed with the ideas of radiant floor hat conversions a few times.
1) You are likely going to need 2 more lines; one to run hot water, not DWH, to the garage and another to return it to the boiler. Even if you can share DHW with radiant heating, you will still need a return path to the boiler.
2) What exactly are you heating? The entire garage, the attic work space? Will the hydronic be embedded in the concrete slab of the garage or the anchored to the floor of a work space above the garage? Either way insulate insulate insulate, concrete wicks heat to soil with nearly 0 r-value.
3) I think MMM has a blog post where he DIY'ed radiant heat, you might want to go back and read it, if you can find it.
4) What will your heat source be? If it is a new boiler simply place it in the garage (assuming it will be above freezing at all times). If it is the existing boiler, do you have a heat load calculation for both your house and the new garage; you could need more BTU than the boiler can provide.
5) I have to ask, why heat the garage? When I had vehicles that needed warmth in colder climate (think diesel) I used a plug in block warmer with a timer rather than heating the garage. If you are thinking of heating a work space . . . radiant floors, as I understand them, are not terribly responsive, meaning you couldn't keep the space just above freezing until you heat out to the workshop and then dial the heat up. Heating an entire garage constantly is going to be throwing away purchased air every time a car comes or goes.
6) If you have lots of time there is a youtube channel call pure living for life, where a couple is DIY'ing their house from site prep to timber framing; while they don't set it up as DIY instruction, they do a decent job of giving an overview. They elected to do embedded radiant heat in their basement/garage. It might be worth your time to see what it takes for in concrete heat.