Ah, not sure how much help I can be...
(1) - No idea as ours is kept internal to our house and the kids' friends bring their laptops round to connect. This is due to our crappy rural satellite internet system. First of all we have a measly 525MB daily download allowance, secondly, even if we didn't, the satellite modem doesn't support port forwarding and they block practically all ports and thirdly, the satellite system is so laggy that when my kids play on external servers, there's a horrible couple of seconds delay between them doing something and it being reflected in what they see on-screen.
(2) Our Linux server only has 1GB RAM and it's never been an issue with 4 players (the most we've ever had connected at once).
(3) See answer to q.1 :( but yes - what you've said should be adequate - and also make sure your DS doesn't go around op'ing too many people...only close friends that he can trust to be sensible.
Also, from experience, do regular (nightly?) backups of their world to ward off the tears when someone thinks it'll be amusing to place TNT beside a home that's taken another player days to build 