I'd just fork over the cash for a paid system as they are much more professional and easy to manage. My favorite is WHCMS. You can get it for around the same price (per month) as what a couple customers pay for hosting.

I agree with MarbleHost, customer support is #1, get something good for cheap that you know will work :-).

I know you say and want free but I would recommend you spend a little and get a lot
whmcs.com is a great billing and ticketing solution for unde $20 a month.
the time you will save will be worth it