Tell them your attorney will ask your material supplier to sign an affidavit attesting to the numbers being true and accurate, under risk of perjury if they misrepresent anything. 

If you're going to be your own contractor you are going to have to learn to squeeze the suppliers hard and pit them against each other.

Most nicer areas have converted to forced air and AC.I personally converted a rental from boiler and baseboard heaters to forced air, mainly because any replacement parts for the baseboard heaters were specialty parts with only one supplier in the city.  

If you are ok with your contractor, supplier, PM, getting back to you in a week or a month when they remember or get around to it, then you can just wait but generally I find that if you want to get things when they said they would be done then you have to be the one to follow up and remind them to do their Job for you.If you don't speak up then you are one of tens or hundreds.

One investment in a rural area from an auto parts manufacturer or aerospace supplier can be a powerful economic engine that transforms the employment and income levels of area residents.

It sounds like they may not even have the capital to purchase and deliver materials (that’s when I’d usually agree to the first payment plus a lien release from the supplier).For timing, I’d lock it down.

What supplier do you recommend here in the Bay Area?

Measure the area of concrete that's going to be poured in square feet, look up a concrete slab (4 inch) conversion chart to get a yard total, and call your local concrete supplier for a price on that many yards of concrete.

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Preferably someone that has connections to a good discount supplier and can get us to a reasonable price.