Empires is really just for the hardcore fans. It doesn't offer much in terms of innovation over the previous title. As for your questions:
1. No. There are zero new characters. However, the 'create a warrior' option has been completely revamped and bulked up to include the option of giving warriors any weapon that is used in the game. There are new areas, about 4 I think. These areas are nothing special though.
Also, areas like Chi Bi and a few others are not actually on the map. You have to meet these crazy specific requirements in order to play there.
2. Empire mode is fun, but it all depends on how into the game you are. I love the whole series, so I can pretty much play endlessly and still dig it. However, it's repetitive as all hell. Basically, it's just you capturing strongholds until either the ruler is killed or the main camp is taken. Practically no strategy is involved at all, despite the claims of KOEI.
If I were you, I would just save up for Dynasty Warriors 5, which comes out in March (in the United States). Not much news has been released on it yet, but it will no doubt be legions better than Empires. All i know so far is that Cao Pi is in this time, the son of Cao Cao.
UYI