Software for planing layouts and documenting wiring etc

I am working on a decent size project now that is going to have a good 50 CAT5 home runs with 20 cameras, automated lighting controls and security. 

Can anyone recommend software for overlaying building  plans with a low voltage plans..  And something that would help make a nice wiring diagram for the telcom closet? 

I use Omnigraffle.
Use an export of the architect's diagram as a base layer, lock it, and then create new layers as needed.

Here I use Microsoft Visio. 
 
Microsoft Visio (/ˈvɪzi.oʊ/ VIZ-zee-oh) (formerly Microsoft Office Visio) is a diagramming and vector graphics application and is part of the Microsoft Office family. The product was first introduced in 1992, made by the Shapeware corporation. It was acquired by Microsoft in 2000.

Visio or similar CAD program. Visio is usually the easiest and most common/universally compatible.

Anyone have a lead on a cheap Visio package for purchase (separately from an Office Suite)?

You can purchase the old Office 2003/2007 suite with Visio new old stock under $100.  Google it. 

Would there be a market for a dedicated application? Sounds to me like a CAD package would be awfully general purpose. How much learning curve/effort is required that wouldn't be if there was a dedicated application that was specifically designed for that kind of thing? And what types of things would you want it to do that now you have to do manually in general purpose programs?

Open Office/Oracle has a free version I believe.