As an old-school draftsman myself, I faced many of the same questions. There are many tools/techniques for converting 2d to 3d, but for the most part, you have to re-draw them. You can take a floorplan and "extrude" it to get a good bit of the work done, but that doesn't create sloped roofs, etc.
I wrote a mini-tutorial about the process here:
https://stonysmith.com/wired/stepvan/default.asp Personally, I use TrueSpace, but several other tools (Blender, Sketchup) can be used to achieve the same results. My
"first" model here at Shapeways was done with OpenScad.
The biggest thing when facing 3d printing for the first time is to understand that surfaces must have "thickness", and that most items will need to be hollow to keep the price down.
Consider these two simplistic "houses". The red one is going to be a solid block of plastic (expensive), whereas the blue one will be considerably cheaper to print. The trick is maintaining the necessary wall thickness (the yellow area) for the material you are wanting to print in. Each material has different thickness rules.
Apply the KISS principal.. KEEP IT SIMPLE to start! Don't jump in with adding bricks and stucco for your first model... Develop your skills and do a few test prints before you frustrate yourself trying super complex details.