Cat Dog, I've been in the heating business for 30 yrs so I may be able to give you some idea options. You are 100% correct in doing insulation. First off you cannot heat a "screen house" and secondly; with the cost of energy today a tight building is vital. Portable heating units like you describe are ok and will provide some measure of comfort. The down side is since you will only fire it up when you are working out there it has to raise up the temp from a relatively low level every time you fire it up.
Another problem with portable radiant heaters is you won't believe the amount of propane you will go through. If it is in your budget then think seriously of a permanent unit for the garage. Depending on various factors your options include natural gas, oil, propane, and waste oil. Local building codes and the availability of different fuels in your area often narrow down the choices.
With the exception of a waste oil heater the choices are generally available in sizes to suit your needs as far as BTU output.
Mike
