have your finalist give you a breakdown, let him set the amounts. then it will be easier to determine if his price for excavation, plumbing & electrical, plastering, etc. is competitive. I would insist on making the payments AFTER each stage is completed, not a minute before. If he/she doesn't have enough operating fund to go from stage to stage, I would be concerned about whether this is the PB for you.

In the event things go south, at least you won't have paid for work not done. Stating the obvious, but please ck customer references and suppliers, ie his gunite source. See that the contractor relationships he claims are legitimate (as opposed to him calling from yellow pages) and he doesn't owe anybody for work they did for him but is outstanding. If he got payments from client, he shouldn't owe anybody, right? Outstanding issues are a big red flag.

chunkochange,
Generally speaking, and this will vary across the country, you should not pay a higher percentage than the work performed.
As a PB in CA , I collected no deposit. I collected about 55% at gunite completion, 30% at deck completion, and 15% at plaster completion and punch list completion.
Some PB's may want a $1000 deposit to get the permit and other minor expenses. Always thought the guy that needs money upfront might be a shoestring operation.
Hope this helps as a guideline. Good luck!

regulated payment schedule in AZ is:

1k down.
30% at dig completion
30% at shotcrete completion
30% at deck completion
remainder due PRIOR to installation of interior finish, which some have issue with when they are signing, but my clients never question once we are at that point and we've earned their trust. We always try to make sure any punch list items are completed before the final finish installation so that the interior is truly the only thing left to do besides startup.

cheers!