Re: Buying used Fortigate - Do I need a Fortinet subscription?
Specifically for learning FGT the second-hand device may be not the optimal option. Mostly they are sold without any subscription available, which means you can't practice URL FIltering, DNS block lists, C&C, so you are left with Firewall, VPN, NAT features basically . In addition you cannot upgrade firmware (EULA compliantly) if you don't have support contract, and sellers mostly sell outdated firmware as they couldn't care less.
For my labs I use either virtual image with built-in 14 days evaluation license ( as I can download images working for Fortinet partner) which also have no subscription based services, or paying per hour on Amazon AWS (1 FGT costs around 1-2$/hour) - there it comes with all subscriptions activated already. Third option is 60 days evaluation license from Fortinet which enables ALL features and subscriptions for virtual Fortigate, but you have to get it via your sales rep with Fortinet.
The only advantage of playing with the hardware box is you can test all Layer 2 stuff - VLANs, LAGs , switching, which is not available with virtual images at the same level of capabilities.