Here is an indepth guide to setting up a Kingpin server using the monkey mod.
It covers setting up a dedicated server using the monkey mod. http://kingpinforever.yuku.com/topic/1816/master/1/?page=1
If you don't want your friends to have to use the monkey mod client then just don't uncomment this line - //kick dirty' in the comp.ini (all explained in the guide).
If you don't want a timelimit then make sure all the timelimit options are set to 0 - set timelimit 0 (again it's all explained i nthe guide).
It might look complex but if you follow the step by step instructions you should be able to successfully set up a KP server running over the internet that your friends can join.