Automated billing and invoice online application (php, html etc..)

We run a small web design company and need an online web application set up to our exact specifics. I don't know if it will be cheaper and/or easier to adjust current open source code or if doing it from scratch will be better in the long run. We would prefer it's done from scratch and that we own it rather than it being an adjusted open source program. I have no idea how much this thing would cost so I need some fair opinions. Even if you aren't going to bid, please give us an idea of the cost. If possible an adjusted open source option and also a price option if the program were made from scratch. This is to be done so it can be uploaded to a secure server and also run on a mysql database. Installation doesn't need to be provided but instructions will be required. We need the program to do the following. * We want to be able to set invoices to be sent to clients on a monthly basis, weekly basis, annually and any other time frame we need... also a manual invoice send. * We need to be able to skin the panel and the front end. * If possible, users can log on to view their bill. (we don't need credit card setup yet but just their balance. * We want to be able to add credit to their balance. * If possible we want to be able to add old invoices and current ones. * The invoices **don't** have to be a generated pdf but it would be nice to know how much extra that would cost. * The system should have settings for New Zealand Dollars (default), UK pounds, Euro and US Dollars... Mostly NZ dollars is important. * We want to be able to export the data to excel * The backend panel should show a list of overdue invoices * The backend panel should show a yearly, monthly, weekly and daily average earning (in a small list) I can't think of anything else at the moment, I don't know how realistic this is and if it's worth it or not. We don't have a huge budget as we have only been going for a year. I really hope someone can help me out with this. I would be very interested to hear your opinions, ideas and of course, prices. Thanks, Dan