Your flash did not work correctly if you are still able to see anything related to RAID options when interacting with the card. Flashing it into HBA mode takes all of that away and requires a bootloader to be installed in order for a connected drive to be bootable.
PLEASE do not just do a RAID 0 on each drive. Bad bad bad bad. You’d be better suited to just doing hardware RAID in that case.
Did you follow the last optional step on Fodeesha’s guide? You need to install the boot loader in order to boot off of any of the drives connected to the flashed PERC. Although if you’re still seeing RAID options your flash may have not worked. Are you still able to get into the PERC configuration page that allows you to create and destroy arrays?
I was able to get an R720 at auction for $70 shipped on eBay, those are the oldest I would go.
If you are going to use it for a game + file server you’d be better served by a newer lower classed model, e.g. R230, R330, R430. RX40 if you can swing it.
Dell is very picky about drive manufacturers / class / etc. You can dismiss the alarm temporarily but it will come back every day or two unless you completely disable that specific alarm in the iDRAC interface.