Congrats on nearing completion on your garage.
If you've taken some PICS of it's progress along the way, start a new thread in Gallery section and show off your build. GJers just LOVE new garage builds !!
For your heater, you'll need to provide lots more information. Like what climate are in ??? Update your GJ Profile with City / State to get best advice.
Also, what level of insulation do you have in walls and ceiling?? What are other heat losses like man door, windows, garage doors, etc?? Will the garage doors be insulated??
General group of hanging heaters fall into groups:
TOP TIER - - Sterling Garage Guy, Reznor UDAP and UDAS (seperate combustion), and Modine Hot Dawg
MIDDLE TIER - - Beacon Morris (Menards), Mr Heater Big Maxx (Northern Tool)
CHEAPEST - - ProComm
Here's an affordable vendor for hanging heaters that will ship FREE. They price match ANY quote you can find nationwide. I haven't bought mine yet, but research led me to them (no affiliation with me personally).
http://www.qcsupply.com/modine-hot-dawg-heater.html
If your garage will be well insulated, a great heater would be Modine Hot Dawg 45K Btu that is $605 (free ship) from QCSupply.