I was thinking about getting the non-EE versions of the series for a friend so we can play together. I know back in the day, the games (at least the 2nd one) were notorious for people having a difficult time trying to connect to each other. We use Hamachi, if that helps. Should I expect a lot of trouble when trying to join a game? Just out of curiosity, can EE owners play with non-EE owners?
If you've tried multiplayer with an Infinity Engine game before, then you should have a good idea of what to expect. The original games (non-EE) are the same as they ever were for setting up multiplayer. I've always been able to get them to work, but it always took some work. Hamachi will probably take care of any port issues you might run into (I think - you might still need to set up port forwarding on the host), but if you have trouble, you can also try Game Ranger. I successfully set up and ran mp Icewind Dale with Game Ranger without too much trouble last summer.
Edit to add:
The multiplayer installs have to be identical. If you are using mods of any kind, you'll have to install the same mods in the same order with the same options.