Can I set firewall on external internet to prevent remote loging and enable remote login at home LAN?
Yes, I think that's possible, although I cannot say for sure because I personally haven't (yet) tried to set up a home LAN. However, I've got some older computers lying around, so it's just a matter of time before I try that.
Easy firewall? A Gnome-based FireStarter is nice and for KDE there's GuardDog. These both have GUI frontend but sometimes it's easier to use a ready-made configuration script that you run from the command line. I use FireHOL -- check out my post on the first page of macondo's sticky thread at the top of this forum.