The V8 Owners Forum -

Any electric heater will be expensive to run. An Infra Red style will probably be the best bet though. Radiators and blow heaters really wont make much of an impact.

A wood burner or similar would be the best bet, although they dont give instant heat. If you google, plenty of people have made DIY wood burners out of old gas cylinders, so they dont need to be expensive.
Or check gumtree, free ads etc

http://www.ludlowsurvivors.com/stove.html

Trouble with IR is it only really heats the thing it hits, good for drying painted bits, not so ideal for a garage. Better off with a halogen really.
Like the looks of that Primus Mike, though not over keen on the pressurised petrol bit