Think 14 days should me more than enough maybe charge more if people wanted more than 14 days.
Managed updates great idea and support is standard (will support be 24/7 or would this be working hours and if so what time zone would it be.
Think you should offer branded and white label versions of the hosting.
Pricing by resources would this be by client or contacts? I always think pricing per client it could be seemed as a negative for growing. Most PSA's are charged by a tech.
Do think on the hosted platform i think we should have an option in the administration for support access where it creates a support user with the email address for example support.access@itflow.com and a ramdom password that would expire after 7 days by default and would create a shareable link for the password for us to send to support. This could also work on the self hosted ones for the customers who have the white label key added to the administration area.