I have already built 2 Client Management applications that are web based for 2 different ISP's. These systems did more than just keep client records and the ability to view them, but I built a full ticketing system, product service system, a portal so the clients could submit their own tickets and modify their services if allowed by the system (i.e. Add an email box, add a domain name, etc.) and all of it automated. I also built a fully automated billing run that would charge the customer every month based on their anniversary date as well as a full taxation system. The automated billing did not just create an invoice and send it out, but if their credit card was on file, would actually charge the card, run through a Paypal transaction, whatever they had on file, and create a receipt and then automatically email invoice and receipt to client. It had a fully operation tickler system so the employees could schedule tasks to be completed.
Im not saying that you system would need to be that complex, just relaying to you the complexity of client management systems I have built before and how easy a simple one would be for me.