This site is not for solicitation of services or 'purchasing' development. Please do not post requesting side mods/plugins and so on. Your thread will be removed, and you will receive a warning.
Hope this answers your question.
ForgeEssentials Being the biggest. if I'm not mistaken it has a way to protect land.
You can browse this for some more http://minecraft.curseforge.com/mc-mods/server-utility
I dont know how to help you with 'authentication'.