Cotisations for a SARL business in France?

My husband, son and I are considering setting up a société à responsabilité limitée (SARL) business in France. Previously we ran an antiques company in the UK, selling French items to the UK market but this closed since the exchange rate declined.

My son and I are separately registered as auto-entrepreneurs businesses in France which allows us to continue our business on a simplified regime, but the minimum turnover is anticipated to become a problem as is the simplified regime which does not allow us to offset costs.

Starting from scratch, without any other income to take into consideration, can you please answer the following questions:

1. Can you tell me the minimum salary which must be claimed by each shareholder, based on equal shares?
2. Can you simulate the cotisations due on an annual salary of 20,000 euros.
3. I understand that in the first three years of trading, annual cotisations are estimated - are you able to tell me what this is based upon and if it is standard, what they are likely to be? Estimated turnover is expected to be around 100,000 euros.
4. Is the above in addition to any corporation tax due?
5. Can you confirm that the margin scheme in France exists and the calculation applicable?