I'm interested already! With the fast leadscrews, are you using a single fast nut for each axis? I've been in machine design for a while, and there are several things I think of immediately;
1) Using a single nut will have a lot of backlash
2) Using a single nut will have a lot of wear, which will increase backlash over time
3) Use two nuts, preferably with a very wear resistant and low friction material (like PTFE filled acetal) that are adjusted for near-zero backlash
4) Fast leadscrews have a higher pitch angle, which means they apply more torque to the nut when turning. This can twist the carriages if they aren't ultra rigid
5) The NEMA17s are good, but they may end up limiting your maximum acceleration. If you can't accelerate at at least 1500mm/s^2, upgrade to NEMA 23s.
More specific to printing;
1) Use a hotend that can't melt and has nozzles close together.
2) Bowden tubes are the way to go if you want faster accelerations and overall faster prints.
3) Use an aluminum heat spreader plate with a sheet of PEI on it. Big benefits!
4) If you enclose it (which you really should) use polycarbonate windows and make sure that all the parts in the hot zone are heat resistant and non-flammable.
Good luck on this build, make sure you keep us updated here!