If you select for your server zombie mode, I think that there need NPC Zombies with them much more interesting (some players hate to play the class zombies).
For Role-Playing server, you will need a lot of time and effort, you need to consider correct economy (to set reasonable prices for the real estate and cars), to consider the factions, to make the interaction between them.
For DM server you need is not so much time, as with all the others. I would advise you to make it different events (races, DM zones, etc).
The choice is yours! Good luck in creating a server!