PhillipAlexander- I strongly suggest Sketchup as that is one of the closest 3D softwares to CAD and the learning curve is very easy. In fact many architects use it. I use it for Architectural visualization professionally among other things like inventions, environments etc. There is a free and Pro version the pro version is not that much different. Pro has more exporting options and some other added features but nothing really crippling to make great models. There are many plugins including an OBJ exporter you can use to get your model into Blender and export as FBX etc. If you can afford it, the Pro version would be better for you though. http://www.sketchup.com/
To go along with this 3D modeling program you can also get rendering software for making more photo-realistic images. For starters I would suggest Kerkythea. It is powerful and free. http://www.kerkythea.net/cms/
There are other render software out there like Thea Render and Vray. Just google 3D rendering software.
I also just discovered Sculptris wich is an awesome free modeling app. http://pixologic.com/sculptris/
I would also recommend getting Blender too as it is open source and very powerful. http://www.blender.org/
Hey graphite! Good to see you.