If you snoop around in a few recent posts on the forums you will notice that Fortinet has removed some FOS features from the smaller units. Some can' t use VLANs, some don' t have IPsec VPN in the GUI anymore etc. Sometimes features still are present but have to be enabled. Sometimes features are configurable in the CLI only.
Unfortunately, there is no central document where these limitations are listed. To play safe and get all the features of FOS you should consider getting a 60D or 80C and not a unit below that.
Ede
"Kernel panic: Aiee, killing interrupt handler!"