Richard nailed it. Ignition sources in basements and garages have forced several code changes over the years. Take gas water heaters for example. They now have to have all sorts of explosion proof safeties on them that have more than doubled the cost of conventional heaters. Garages are a particular problem as people don't realize how much flammable stuff they store in them. Gas cans, paint thinner, cars, lawnmowers, etc. These all can be potential flammable vapor sources.
You need to verify if the heater you are purchasing is rated to be installed in an enclosure. some are only rated to be installed outdoors. as far as venting goes, you need to look at both exhaust and intake. an improperly exhausted unit can damage it not to mention fill the room with CO