The first thing you should ask a potential host is whether or not they support SSL at all. You'll need a dedicated IP address in addition to the SSL cert. Different hosts charge differently for IP addresses so make sure you find that our as well.
If you're planning on being a reseller you should find out how you will fit into their system. With H-Sphere resellers can either be located at reseller.theirdomain.com, or reseller.genericdomain.com. Going with a generic domain name will help separate you from your host. Also, find out if you'll have your own virtual nameservers.
I am not sure how other systems are setup, but HSphere does the billing completly itself, no extra software is needed. Other control panels will be different.
So, to sum this up, you'll need a cert for your website to accept credit card payments securely (for things other than hosting and possible hosting, depending on the control panel), and you'll need to find out what control panel your host is running.
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