I think you should create two pages - a quote form, and a payment page (NOT included in the navigation). Here is the process I have in mind:
1) The client fills in the quote form
2) You read over the quote and decide if you wish to work for them
3) You then e-mail any successful applicants saying congratulations, and giving them the payment link in the email.
With this way - Your email is the only way they can get to the payment page - and it means you still get your freedom and they can still pay for the work upfront.
Just a suggestion - as for the marketing, if an idea was that clever, someone would already be doing it - or wouldn't give the idea away, sorry.
Luke
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