3D printer beginings

Ah today I started to put together what I want in a 3d printer. My goals for this project are as follows.

1. 100% modular design. I want to be able to extend the bed, extend the height, change parts, at will. I don't want to be locked into anything if I can help it.
   
2. 1 Print head for now, but I need to be able to add a 2^nd at some point in the future.
   
3. Ball screws for the Z axis (nice and smooth movement)
   
4. Larger format than most printers I've seen. Looking to get at least 12”x12”x12” total print area. The printer I'm looking at looks more like I will be about 18”x18”x21”,
   
5. Other than that just fun and design. I am going to start with a basic design and I will go from there.
   
6. I will be using the plans from http://openbuilds.com/builds/c-bot.1146/ as my starting point
   
7. I ordered 10500mm of 2020 aluminum today. That will be used to build my frame and the x/y gantry.