Technically you can. The game checks the legal serials only when you connect to another machine that has Dom3. So mostly just when you get into multiplayer games. Then you have to be careful about possibly playing the same game from multiple computers. The game would only see that the serial is coming from two different machines. It would have no way to know that they are both really you.
But for solo play and tutorial (or running a copy as a server) its no problem.
As for race, Im not sure. I tried awhile back to get an agreement about what race is simplest and already basically understood by non-players. But as you say, everyones experience with this game is different so it was alot more discussion than agreement.
My thought was something like Middle Era Ulm. Its strength is basically heavily armored men and cavalry. It has a priest and a mage and a scout but nothing too far from what you might see in any fantasy based game. To me it seems one of the more generic recognizable nations. I think it gives a chance to learn how play the game without wondering how to play Ulm.
Now that might actually turn out to be your perfect match. But if it doesnt, dont sweat it. As you play with the others you will be able to match nation to your style (offensive, defensive, turtling, researcher, stealth, horde)
And welcome to the addiction.