If you do not already have a ticket system in place and do not have a billing system, then WHMCS is strongly recommended from my end too.
I've used many different ones over the years - Cerberus, Kayako, InteliDesk (I think - they've been out of business for 7 years now), ModernBill, PerlDesk and others. Each one had their functions, and Kayako and Cerberus were perfect for a business with multiple operators and departments. But my choice still comes back to WHMCS if you're starting out and want a billing and support system in one.
I'd also go with a helpdesk rather than providing support via email. Similarly I'd provide live help on your site rather than using MSN or AOL or something.