If you are serious about being a hosting business, then get your own servers in a reputable Data Centre. VMs are OK as long as you control the hypervisor.
From the questions you are asking, it sounds like you have a steep learning curve in front of you. I will try to answer your questions quickly, but for all of them there is a much longer answer that I don't currently have the time to type!
1. Probably not (...and what actually is an SBC anyway
)
2. Yes, but if you are controlling account/phone provision you could use a different port, a port is just a port at the end of the day. If you do move away form 5060, you may need to change firewall and fail2ban rules.
3. NAT should not be a problem FreeSWITCH handles it very well.
4. Fusion will handle 300 users with no additional performance tweaking.
A lot has been written about scaling. Nearly everyones situation is different. Learn how to monitor you systems performance, things like checking receive buffers, analysing packet captures and looking for SIP re-tries etc. There are a lot of standard Linux tools out there that will help, you don't need fancy software.