Don't want to dampen your enthusiam on this one, but the first thing that jumps out at me is why have they not been cleaned for years? Are you likely to get repeat business from this or just hassle?
You say they have already asked you about your price, so how have you been able to quote it if you don't use WFP or have you considered cherry picker access? You must have had some idea what method when you put your price in.
If you get bad results:
Did you explain that due to the fact that they haven't been cleaned for years, and that the framework has not been maintained that results will be limited
How do you get paid:
What are your terms on the quote, have they agreed them?
Someone working the insides to check they are clean:
If the business will allow that to happen, do you want to pay someone to do that?
Standards expected:
Have you asked what their expectations are?
(By the way picky housewifes pay your bills
)
Personally under these circumstances I would walk and have a happy hassle free life thereafter