I would use pay-as-you-go model which lets me scale as and when required and pay accordingly.
The traditional problem with Home grown software is you can't accurately predict the growth when you open the floodgates. So choosing a pay-as-you-go model works perfectly here.
The other constraint I see with Open source software is most of the them expect a experienced user and clearly lack an established support system which you can get in a paid software.
I would be more helpful if I understand the details in depth and happy to discuss that over a call.