time, expense and billing script

Am looking for a timesheet script including expenses and billing of these hours and expenses, with a possibility to work with prepayments. Must include employees(users), clients, projects, activities(with rates). Must be able to make a billingproposal with the possibility to make corrections, before the billing is definitive. Have a nice opensource example (Timetracker, but that has to be changed. Any other idea or script is acceptable as long as it is simple without to much other fuzz around it. Please only serious coders, because I had already a bad experience with this.