I have not personally done what you are asking but I can help with some of your questions.
First of all, Fortinet has a very good service, FortiConverter which can convert configurations from other vendor Firewalls to FortiGate configuration.
Once configurations are imported onto the FortiGate, it can be imported into FortiManager for central administration.
SD-WAN scales out incredibly well. This is because of the way Fortinet approaches SD-WAN. Fortinet does not offload the control plane from the Fortigate. Each Fortigate in the network is acting as its own local SD-WAN controller. There is no cloud-based or centralized controller that needs to be sized for the network.
The only real sizing consideration you need to make is the hub will need to support the number of VPN tunnels for your spokes. These numbers are readily available on the data sheet.
Speaking of data sheets, Fortinet has some of the most accurate data sheet numbers in the industry. They can be trusted! Just keep in mind the SSL Decryption numbers are for that process only so if you are doing other features like VPN, NGFW, etc the numbers become lower collectively.