[OCCOBRA]

Our PE at the main office cinfirmed my thoughts on your sizing. It appears that you will have a 25,000 btu/hr load given the factors listed above. Beacon Morris (one of the companies owned by Mestek, my employer) builds a Horizontal Unit Heater Model HB125A which is rated @ 24,800 btu per hour @ 200 deg F entering and 180 deg F leaving. Since your boiler is 160 deg F setpoint you will recieve 23,882 btu/hr output when the water is hottest, and 16,516 btu/hr when the water is @ 120 deg F. As an average say at 140 deg F boiler water your output from the heater would be 20,187 btu/hr with a cold garage (30 deg F) When the room heats up you will get 14,161 btu/hr input when your garage is @ 60 deg F which will keep you nice and toasty. They max flow rate through the unit is 2.5 gpm with a head loss of 2.2 ft hd. This will only utilize a very small zone pump depending on the run. Your loacl Hydronic supplier should be aable to help you pick a pump. I currently do not have a used R & D unit. Apparently these disappear fast after testing.
Check your local supply house, distributor for our product or use another brand if you can find a good deal (you wont hurt my feelings).
If you have an old radiator that is laying around you could use hose to AN fitting adapters (Jegs/Summit have them) and use hose to connect AN/NPT fittings to your existing piping, then get a fan that will do at least 500-600 CFM to blow through the radiator. For a car guy that would really be cool and would/could be cheaper. Just a thought.
Hope this helps. Any other questions and PM me
Dane