I have been working in online and offline marketing for the last 6
years. I have seen many companies try and build their own 'in house'
intranet / extranet.

Usually they estimate that it will take 3 months - or even less if they
think they can use Sharepoint or similar tool.

However what really happens is that it takes closer to 1 year to geat
something operational - regardless of platform!!

Do yourself a favour - use a pre-written system from a commercial
verndor.

Have a look at:

http://www.HotProject.com/
http://www.basecamphq.com/
http://www.aceproject.com/
That way you can get up and running instantly!