Robertphil,
Firstly, don't mention to any commercial that you intend using ladders on their premises, that's the first bit of advice.
Supply them with all the correct paperwork and insurance documentation.
Do a good job, and use them as a reference if you quote other work.
Think bigger than you actually are, but be sensible in what you think you can clean.
Just be confident but know your limitations.
Ask the commercial you are cleaning do they know others that may require your services.