I wrote a customer billing, administrative, ticketing and sales system for a small ISP (that ended up growing into a larger hosting company). The system integrated with the email server, RADIUS server, vendor ticketing systems a web portal for clients, had it's own inventory tracking system, IP allocation tool and managed the sales process from lead to quote to billable account. It is definitely doable to write your own but keep in mind that this does require some commitment of resources to not only write the software but maintain it in the future. If you want an integrated system, writing your own is the way to go and developing those systems while you are smaller is much easier. In general commercial ISP management systems are either too expensive, too rigid, or too fragmented.