Don't buy any one of the fcs. They are outdated and won't be supported by latest firmware releases of inav.
If you can spend money , get a f7 or higher fc. F4 is a good option but the devs have stopped releasing firmwares for this fc.
So better to invest on a good fc.
First tell us which application you need the FC for ? I can provide F3 FC to you for free. But they would not be supported as Surjit says.
Latest Betaflight has old Firmware for most F3 FC or have many new features removed for them. iNav Boatrd support is very sketchy and you will have to first check iNav supported board list (form its Hex release page) if the board is supported. Many F722 and hihger boards are not supported. CleanFlight will support almost all the board in its latest or old release.