Its all been done before.
For a hundred bucks you will find that you cannot even afford to drive past the aircraft in question let alone wash it.
Private owners generally wash / polish their own.
Charter only pay when there are no budding " wanna be's " hanging around.
Then you have permits / licences / permission to trade from airport owners.
ASIC's
EPA pop their head up. No residue on the ground ect ect.
Insurance and maybe issues with log book entries for some of your work.
Workers Comp / vehicle / pump / vac / water and whatever else.
Go for it, should be fun.