Ask HN: What to use to build an intranet/extranet - OSS CMS or DIY?

I am an experienced programmer developing my own startup and am doing some part-time go-tech-guy consulting.

My boss wants some kind of publicly available, secure site for hosting internal documents (like hr docs and company news) and links to webapps used within the company for about 200 people in different locations.

He would like a groups/permissions model to control who can see what documents and what links they get.

Should I a/ install an off the shelf CMS, and if so which one? This is a business which mostly uses .NET as a tech (and indeed I am a highly experienced .NET) dev if that makes any difference.

b/ As a developer with some nifty RAD frameworks I can knock together a simple landing site with authentication/groups pretty quickly. Install a simple blog engine as well and I'm done?