Joining one of the worlds after the fact won’t be a problem. In fact, it will probably be beneficial to you because economies and campgrounds and guilds and the like are already established.
Assuming I can’t talk you out of attempting at least one of them (your month or two could easily mutate into an extended period, depending on your personality), I’ll give you my advice on the current games as I know them.
EQ I played on and off for about 2 years and it’s probably your best choice of the lot of them. It’s been around for what, 3 maybe 4 years already, undergone several expansion set integrations and has a very well established economy (in game as well as out). Their class balance, which while leaving something to be desired, isn’t nearly as bad as other games.
DAoC I played for about 6 months straight, shortly after it came out, and from what little news I still receive from old friends and guildmates, the game hasn’t improved all that much. They took steps to better a lot of the things that EQ players detested, and in certain respects they did wonderfully, but overall the game lacks a sense of purpose. Sure you can level up fairly quickly, and exploring the world isn’t nearly as dreadfully time consuming (which has its downsides, as when looking for a spot to hunt that isn’t overcrowded), but Mystic seems to have focused too hard on getting everyone to level 50 and forgot to add level 50 content. While great ideas, keep taking and relic wars and the like didn’t even approach what they could have and the game was so full of exploits that it was hardly any fun for anyone at that point. Logical strategies fly out the window in the face of an invisible archer hiding in a wall killing people in one shot from a distance best left to guided missles. And don’t even talk about class balance, there is none.
Shadowbane I have not played, but I’ve spoken with several friends that have been involved with it since beta testing and none of them are impressed. Especially not to the level of which the game has been hyped for the last year and a half. I’ve had it described to me as a glorifed Diablo II, but not as much fun, so take that as you will.
AC2 I have no direct or indirect experience with, so you’re on your own there. I knew a great many people that thoroughly enjoyed the first version, so if it’s half as good, that might be a good shot for you. However, it was out at the same time as EQ and EQ dominated the market. Good reason for that I’m sure, though I know not what it is.
Personally, I’ve given up on the MMPORPG’s and would try NWN. There are single player modules and multiplayer capabilities, but if you do not know people to play with, you’ll likely end up in similar situations to your Diablo II experiences where you’re playing with a bunch of d3wd5.
Good luck and good gaming.