I suppose I have to be the advocate for ventless heaters.....*sigh*
I am doing exactly what you are considering in a 22'X30' garage using my Procom 30,000 btu natural gas ventless in the Chicago metro region.
I have no condensation issues at all and I keep the space at 55F during the cold season and bump it up to 65F when I want to work or putz in the garage.
If you have the regulator adjusted properly for the right inch of water column, there is hardly any smell....though my garage isn't super tight.
I also constantly run a cheapo box fan, 4 foot above the furnace, blowing the heat across the space and constantly keep the air in the garage circulating.
It is insulated with rigid foam about R13, if that.
I paid about $200 for the heater with the optional blower fan and I'm paying about $40 a month to heat my 660 squares.
Having a heated garage in the winter is AWESOME!!!!