If you are targeting the mainstream reseller market having WHMCS is a must. You can offer it along with CE. To be deferent you may also consider offering free installation etc (takes 5 min to install from shell).
Another thing is End-User support for your resellers clients (this is also a growing requirement).
RvSite Pro is probably the best choice when it comes to site builders.
Another option is to offer DirectI domain reseller account.
Well that's all I could think of for now
