Here's my opinion. I'm not an engineer and don't even play one on tv.
The exhaust heat exchange is trickier than the coolant heat recovery. If you cool the exhaust too much, you will get condensation of some fairly nasty and corrosive stuff. The presence of that moisture will cause soot to stick like mad to the inside of your exhaust pipe, rapidly clogging it. Thus, some means is necessary to monitor and clean out said exhaust pipe. Other wise, lovely. You could also just have a longish (not too long of course) exposed steel pipe and heat the garage directly.
Commercial outdoor wood furnaces are, as a rule, pretty inefficient. If you actually capture 35% of the heat in the fuel as useful heat indoors, you're doing a marvelous job. By comparison, EPA rated wood stoves may approach 80% efficiency. The commercial ones are also pretty expensive and can smoke like a bandit. No way you could run one in town IMHO. Smoke = pollution too.
If they say "efficient" but don't mention a % or real data from an independent testing agency, it's just hype. That doesn't mean you can't have one, or shouldn't have one, I just don't want you to be dissappointed. If you google search for outdoor wood furnace efficiency you'll learn lots of interesting stuff.
All other things being equal, you will have lower heat loss from a water storage tank that's cube shaped vs shaped like a shoe box because it presents less surface area per gallon for heat loss to occur through. The best would be sperical, which is not a trivial task for a homebuilt tank. The taller the tank, the stouter it will have to be constructed. Don't even think about pressurizing such a tank.
Heat exchanger sizing gets messy. Depends on the material, size, flow rate, construction and some other stuff. Temp in and Temp out don't give quite enough information either. We'd also have to know the flow rate, which would give us BTU/hr. That's really outside of my specialty though.
Yes, things can grow in warm water tanks. My particular design is also bad because the water in the tank is stagnant, serving only to store and release heat. There are biocides you can add to forstall that and it really depends on temperature. You can also get epdm liners with built in algaecide. Better be sure you're not drinking that stuff though.
Good luck, have fun, be careful!
troy