KVM gives bare metal experience and OpenVZ 7 is another light weight option and I would vote for SolusVM as it is more polished and mature then many other panels
There is no any single doubt that KVM based virtualization is best as hardware is utilized much efficiently.
There are lots of software in market (free and paid) to ease the process of virtualization like Proxmox, Vmware, VirtualBox, etc.
I would suggest you to go though these first before digging into know how of automatic VPS provision.
Your concern regarding billing system is not clear to me. You can integrate Whmcs on your website for billing.
SolusVM is virtually dead. There haven't been any real improvements in years and I have used them for years. I moved to Virtualizor. Both work with WHMCS for automation.