Are you looking at providing "Exchange" email hosting, or just Linux (Exim/Postfix/Sendmail) type hosting?
As realistically you could do this from a fairly simple VPS solution, just tweak your control panel skins to hide the hosting options and disable apache/litespeed.
ConfigServer make some great anti-spam in and anti-spam out plugins for both cpanel and directadmin, then there would be far less overhead.
DirectAdmin handles IP per customer etc, which I would recommend.
If you are looking at reselling "Exchange", then OVH is the cheapest provider I know of that I would trust, but you will probably have some API programming work, as I can't see an OVH Exchange plugin for hostbill or whmcs.