At 10gpm, you'd likely need two tankless units in series at very significant cost. With the boiler upgrade, I would seriously have considered an indirect. There are many direct vent gas WH available that don't need a flue through the roof. How cold does the garage get in the winter (ever leave the door open for awhile?) and how cold does the incoming water get in the coldest part of the winter. These all play in what size you need, and whether a garage is a good idea for water lines in the first place.