The 80F is a fantastic firewall. First thing to keep in mind is that it is a 'desktop' form factor so will not mount in a rack unless you purchase the rackmount kit.
To answer you questions, yes it supports link aggregation. All FortiGates support it: https://docs.fortinet.com/document/fortigate/7.2.1/administration-guide/567758/aggregation-and-redun...
High Availability with FortiGate is mostly done using Active-Passive configuration where configuration and sessions states are synchronized from an Active 'master' node to a Passive 'secondary' node. If the master node fails for any reason the secondary becomes active and continues with traffic flow.
Active-Active is rarely used as it only load-balanced the security inspection between firewalls. Typically you would see this in areas where you require very high amounts of throughput for security inspection (IPS, AV, etc).
If you're talking about price you can just ask your reseller what discount percentage you are receiving. Use that to figure out what list price is. :)
And yes the FortiGates come with an amazing amount of features and functionality built in with no license requirements including: SD-WAN, VPN, App Control, Wireless LAN Controllers, Wired LAN Controllers, etc, etc. FOrtiCare support, however, is a paid subscription along with FortiGuard services like IPS, AV, Web Filtering.
If you are running a stable FortiOS release like 7.0.X you should be OK with stability.
I would 100% recommend the FortiGate. I was a FortiGate customer 7 years ago and made it my mission to work here as I recognized what an amazing product it is.