DIY 450x450x900

Hello everyone!

I bought my last printer and I have been using that since 2014. I am ready to upgrade and would like to build my own. I have been making decent progress on it, but can't seem to understand the mechanics that get me past a few areas. The print area I am aiming for is 450x450x900(approx. 18x18x36). I had planned to move the bed vertically and mount the print head at the top. I currently have was made by cohesion 3d and is a gantry style printer. What I am not sure about is if I want to go corexy or hbot or something else yet. What does everyone recommend?

I know that I want to auto level the bed, after 4 years I'm over doing it by hand. Physics would dictate that would be 3 lead screws in a triangle to have a bed that levels in both axis. I saw most of the designs use 2 lead screws, so how do you account for the second axis on the bed with two?

If I do use the triangular design, then that is 3/5 stepper motors right there, with 1 for x and 1 for y axis. So how do I add more motors so I can actually have a few extruders(3-4)and not just a moving gantry? I was planning to print nylon, abs, and pla. The last two in different colors simultaneously. I was hoping to do 3 colors and a support material such as PVA.