To be honest, I would not recommend Proxmox. Or at least, you will need a paid licence of it (we've got that in my company). Proxmox can be very buggy sometimes. Try something like Hyper-V, you will not be disappointed.
Otherwise, when someone got the idea of creating a VPS hosting service, there are always the same softs used: WHMCS for the website (with a crappy template seen everywhere), Stripe/Paypal for the payement gateway, and Proxmox as the hypervisor (because there is a easy module that is sold for WHMCS that can manages very easily a Proxmox hypervisor).
You will need a lot of technical stuff in order to create your service. You will also preferably needs to contacts ARIN/RIPE in order to becomes LIR and got at least a /24. Then you will need to find a datacenter and a transit provider, etc etc.
The hard thing will be the DDoS attacks. On my side, I paid a lot in order to have a DDoS-protected incoming transit. It starts at 100-200$/mo (with like 100Mbps at 95th percentile) and can easily rise to 500-1000$/mo.
Sometimes, some hosting providers can be totally took down just by one attack (it affects all the customers). If you cannot pay a DDoS-protected provider, you will need at least to do RTBH.