You'll need to use a version of ESX to do that, not ESXi, in addition to not being within the licensing you'll run across annoying problems like not being able to clone machines / manage backups / move VM's between machines. If you pay for it, it's a great system although I wouldn't want to try and setup hundereds of VPS's, I haven't seen anything that integrates with it so it'd be pretty manual. If your going to have a few customers with barely any churn it maybe worth it.
So in conclusion what would you recommend for use with hosting both windows and Linux servers from one main node?
XenServer can certainly be used to host both Windows and Linux on the same machine, assuming of course the CPU supports native hardware virtualization. KVM too.
Being a legal/certified VMware Service Provider is very expensive and only for those with significant financial resources to back it up and the requirements are pretty steep. The minimum monthly payout to VMware for the privilege of being a VSPP is breathtaking for small providers.
VMware actually has some good commit levels. You need to have at least one VCP (~$2700 for the course), and your lowest minimum monthly commit is $360. Their pricing is per VM, and would likely be viewed as expensive for many, but aren't bad considering as long as you aren't a budget host. ~$10/VM for ESX standard, $26/VM for Advanced, and $36 for Enterprise.