Received an offer for my shell construction

The statement from the shell builder was that this was not his first time building and that he can estimate everything with 99% certainty...

I basically find that likable. A man of his word.

Why pay a presumed professional company a lot of money for nothing when many things can be done cheaper and possibly more directly.

Even the more expensive offers will end up costing more, but with the cheapest offer you have much more leeway from the start.

Try to ensure a fair and realistic breakdown of the installment payments in case something goes wrong, so you don't end up overpaying too much.