Space heater for garage...

Electric/oil heater with a built in fan. Get a couple of them. Leave one of them running 24/7 when it's cold outside run the other only when you're going to be out there.
 
get one of them barrel fire boxes or whatever their called , and then just run a stove pipe out the side of the shop , thats what i did , and its sometimes to hot , and they aint to expensive , its probably the best bang youll get for your buck if u got available wood to burn