Hi Jim
That my friend is what i call a good idea,
Firstly, four months ago we needed a plasterer, not urgently, we made a small loft conversion, needed plastering at some point, it was only when I saw a car, outside a supermarket, sainsburys, with signs inside each window, printed, not scribbled, for a plasterer, so we called and hired.
Cant remember my 'secondly' point, but a sign written with quality graphics etc, with marine ply upright facia's and a discreet painted up trailer wont cost alot, my 8' 5' trailer for my other business cost me 160 quid on Ebay, its a bobby dazzler, so that coupled with two sided decals and structural wood (shopping wisely) will get you that mobile advert 6x6 ft outside your leading supermarket for under 500 quid.
Go for it
Dan