1. No, missions dont have to have an objective. Thats just info for player. Trigger type end1, end2, end3 and so on make missions end.
2. Standart port number is 2300 I think, but it is using some other ports also, as I saw of my Sygate firewall, that doesnt help opening only that port. Maybe I'm wrong on this. Also a game server can specify other port than a standart one.
3. Resistance is not only a pack of addons but also an update, so getting just addons wont help. I'm afraid, you have to buy it.