We used to do the DIY 3D printing kits a few years ago. We did the MendelMax design, then did our own design. Played with Rostock too. I was thinking to look at this again, now that I know more about cnc and have better tools available.

The extruder for me is the key thing to get right, then the bed / heating setup. The movement (XYZ) is almost the easy part.

The Ultimaker 3 shows what can be done in the 'DIY' market but I cant see the value in the cost of that.