It's certainly easier with a hot box,
You will most certainly need a pressure washer to get the grafitti off the walls.
Try greenhams online, you are look for selden k14 (I think), its about £6 an aerosol can.
Depends on the amount getting off and the colour the surface to how much you'll need.
It does a good job also will clean pen marks etc of all types of surface.
I clean a few schools with this stuff, good on the outdoor huts, vinyl type ones, metal etc.
Alternatively to get a truly good job albeit messy get a sand blast attachment for your pressure washer and use kiln dried sand. Works even better.
Darren