My understanding is that FreeNAS is a complete OS and you would not combine it with Ubuntu. Or you could run Ubuntu and Samba and not need FreeNAS.
FreeNAS will work without a hardware RAID, using software RAID. It would seem that this would be easier overall. Ubuntu can do the same thing with the appropriate add-on.
Linux servers typically don't need very much in the way or resources and can run very well with a lot less CPU and RAM than what you have. And yes they are capable of running for a long time without a reboot.