We could make a limit of 50k on members? Too high, too low?

As for interest + loan repayments, the conditions could be that the person has to return at least 10% of the TC each week. For interest, we could ask for a flat rate of 10%, payed once. So if someone borrows 20k, then they pay back at least 2k each week, plus an additional 2k once, as interest.