Hello there ~
Just in case, the deadline and price are placeholders, for now, until we can discuss the details of each thing you need, since there could be some factors that will increase exponentially the complexity of the system.
I've been working on creating and mantaining part of the accounting system in a company, which involves invoices, receipts, receivables per person, due dates, interest, taxes, payments, and many other things which could increase the effort, time needed, and of course, the price.
But well, as I said, just ping me out and we can talk on this further :P.
Stuart.