Respect the first question what I think it is that for proper forecast of the convenient foundation, and hence any likely cost estimate, the concurrence of a structural designer is a necessity, if none of the parties gathers the required abilities.
Respect the second question, excavation in general, you can consult for example
Moving the Earth 4th Ed.
Nichols and Day
Mc Graw Hill
Standard Handbook of Heavy Construction 3d ed.
O' Brienm Havers, Stubbs Jr.
Mc Graw Hill
Respect concrete, in developed countries it is unlikely that for structural purposes reinforced concretes of less than specified compressive strength 25 MPa be now used, since cement contents -correlative to strength- is very significant in the protection against corrosion. However for poor cement content and mass concrete boulder foundations, or preparation layers, lower strength concretes are used, maybe up to a minimum of 10 to 7 MPa, but more typically in the 10 to 15 MPa range.