You should go for linux/iptables.. I run a linux (first 2.4, now 2.6 kernel)gateway at home since a few years and never had any problems.. I first had a 486/120Mhz with 2 ISA NICs and it also worked, although I did ugrade it a bit from time to time.. Of course you can also operate your linux box via ssh at any time..
I don't really understand why it must be easy to operate for people with little computer knowledge. The firewall should be set up by the admin (you) and not be changed by simple users like grandparents etc.. Maybe some distros offer a gui for iptables configuration but normally you configure it via command line(script)..
