Thanks all!
I am planing to use natural gas, last weekend did the black pipe to bring it out from my house to the attached garage. Right now I am stuffing insulation above the ceiling. I was looking at the garage heaters that are available locally at Home Depot and Menards. It seem these are all unvented - that is what prompted my question - but they are lightweight and easy to install at the floor level. Also got recomendation from heating guy to get Hot Dawg made by Modine. He was recomending the 6oK BTU unit for my 3 car garage. This unit has intake from inside but is vented to the outside. The unit is 80 pounds so it would be a instalation chalenge to hang it from the ceiling. Like always, everything has pros and cons....Decisions,decisions.....