Although, if you are playing on a LAN it's silly to use QuakeWorld. QW was designed to make it easier for people with high latency internet connections (i.e. dialup) to play over the internet, by adding player prediction and delta compression. However, this makes the physics of the game feel kind of whacky.
If you and your pals are just setting up a local network to play, stick with regular Quake (aka NetQuake). If you aren't all playing locally, but over your internet connections, you may want to use quakeworld depending on how much latency you all have (how high your pings are). The other reason you may want to use QuakeWorld would be if you had more than 16 players, as that's regular Quake's limit, where QW's is 32.
However, ctf never had much of a following for QuakeWorld, so any mods available for it probably wont be very mature.
If you do use regular quake, ProQuake makes the best server:
http://www.planetquake.com/proquake/
Qrack is the best client:
http://www.quakeone.com/qrack/
As for the mod, as Baker said crctf would be a good choice.