Very difficult case. As many have already said, this belongs in the hands of specialists. You are certainly well advised with a lawyer specializing in construction law as well as insolvency law.
Important for me would be, firstly, the down payment already made. This is certainly not worth 15%, and claims should be filed against the insolvency administrator.
Further the question is, who will be your future contractual partner? Who is responsible for any defects, warranty? If the new general contractor acts as a kind of subcontractor of the insolvent company and you receive the invoice from the insolvency administrator, this can become complicated in later warranty issues. Here I would definitely have the payment agreements reviewed and possibly consult a building expert during the construction phase. In case of defects discovered later, the whole matter will certainly be challenging otherwise...