Claudio - couple of questions:
Why do you need to replace your firewall?
Is it dead?
Out of warranty?
Or has your network outgrown the capabilities of your existing FortiGate?
To that point, your "network" isn't even vaguely described.
Is it a home network?
SOHO?
Large office?
How many users on-site?
Off-site VPN users (if any)?
What is the capacity of your Internet connection?
Internet service type (DSL, FTTH, MetroEthernet, CableModem, ISDN?)
How many discrete zones do you require?
Do you need WAN redundancy?
Do you require the FortiGate subscription options (A/V, IDS/IPS, Content Filtering)?
Does the device need to support HA?
What is your budget for hardware replacement, extended warranty, and subscriptions?
Have you considered open source alternatives? For example - in my small business office, I use a pfSense firewall that I built on a barebones SBC I purchased at AliExpress for less than $150 - fanless, small form factor box, dual Gb Ethernet (Intel) and a Celeron class CPU. I added an IBM SATA II 30GB PCIe SSD ($30 on eBay) drive for a boot disk and local syslog; and 4GB of 1033 SDRAM ($40 on Amazon).
With 4 office PCs going full-tilt boogey, and INSIDE traffic segmented via VLANs on an enterprise class switch such that all traffic flows through INSIDE zone-to-zone traffic passes through the firewall; AND a 'household' VLAN for 'recreational' network usage (On-Demand video, kids' online gaming, etc.) CPU and RAM utilization on the box rarely exceeds 20%-30%...it's not even working up a sweat.
Our broadband service is 50Mb/10Mb (down/up) - and we use a lot of it during working hours.
pfSense is ULTRA configurable and extensible...via the Admin GUI you can install 100's of stable optional packages - caching DNS, Snort (with paid subscription offerings around A/V, IDS/IPS, website blacklisting, anti-spam filters, etc - every bit as robust as Fortinet's) - some require a bit of configuration before you can start their related services - but again, if you're a FortiGate admin, none of it should give you too much trouble.
And to answer your last question regarding traffic metering / monitoring - there are plugins for that too that present nice dashboard-type graphs of utilization, events, etc.
Online help is plentiful at pfsense.org - and if you do need professional help, there are paid options to get install / config support from the dev community at reasonable hourly rates.
Good luck!
