Well, as bqinternet was hinting at, if you are going to be doing this for businesses then you will want a proper billing solution (and I have yet to find a free one). Have you ever had a billing problem with somebody? Phone company, cable, internet, charged too much at the local McDonalds, etc... it's definatley not fun.
ClientExec is nice, probably one of the better ones unless you feel like spending a bit more on ModernBill.
As above, I'd steer from a free billing solution unless you're offering free hosting accounts, in which case it really would not matter as much. When it comes to your business, whether you're a reseller or leasing dedicated servers/colocating, your image is everything.
While the average user may not look at the billing solution too closely, businesses may.