map pack: $5 for seven extra maps. dlc bundle: $10 for survivors, onslaught, predator and i think siege. no dlc characters for purchase, unless under re.net where you would use points gathered by playing the game and logging in.
install it into your system and it drastically cuts loading time when playing single player. it takes about 10 - 15 when online.
it's pretty alive on xbox, and since psn usually has more players playing online since psn is free (i'm assuming that's the reason), then it should have people playing as well. not to mention the dlc modes came out just a month ago or so for ps3.
i haven't read anything about gold edition, but if that's coming out soon, i guess it'd be better to wait for that.