Lead screw holder - shall I buy metal or just use a printed one?
Update:
To answer your question, you could use either metal ones or printed ones. Metal housings are way more heavy that printed parts. The housing you refer to is not attached to the platform, but a static part connected to the frame. Weight is not an issue, stiffnes, strength and temperature stability should be of higher importance. 

For metal housings attached to the Z platform weight may be an issue (if have many microsteps where the incremental torque may not be enough to raise the Z platform). Printed parts can be made stiff enough to house the linear bearings. In my experience printed bearing housings can be just as effective, I use those on my platform for my CoreXY printer as well as using leadscrew and linear guide rails brackets made of printed parts.



Old answer, before question update:
With platform movement (or printer head Z movement for Prusa designs) you face a few challenges related to the quality of the parts you buy. Note you want smooth operation of the platform (or head) without wobbling. It is customary to add linear rods or rails to guide the platform up and down, this is their sole purpose, therefore these rods need to be very straight and bought from a local trustworthy vendor (the Eastern oversea specimens are usually of less quality as I know from experience). Securing these linear guide rods could be done with printed parts, the plastic is stiff enough to hold the rods in place, and temperatures are usually not that high to play a large role (if so like in boxed up printers, print in high temperature resistance material), personally I use black PETG. 

Secondly, the drive of the platform. Note that leadscrews are not perfectly round, nor is the coupling 100% in the center of the screw. From a mechanics point of view you should never constrain the leadscrews at both ends. This results in an over-constrained (indeterminate) system (of forces) that can induce even more problems. Optimally you fix one end close to the stepper, or the drive of the belt), it is arbitrary whether you use a metal or a plastic part for that unless there are constrains on size and thickness for a requested rigidity and leave the opposite side free. For Prusa clones I use lifting parts that house the lead screw nut rather than embedding the nut in the x-y idler coupler (this separates eccentric x-y movement from z movement and a handy advantage is that if something goes wrong in z min direction, the head will not destroy the glass or bed as it is not fixed to the lead screws). Something similar can be done for your platform. For my CoreXY however, I have not done so, it uses 4 linear rails of 12 mm and 2 leadscrews. PETG stepper mounts at the bottom drive the leadscrews using plum, not spring, couplers (the springy types should be avoided or a ("fixate-able") bearing or KFL08 mount should be placed at the other side of the driven side of the coupler and correctly mounted to the frame). Furthermore, I use Delrin or POM anti backlash nuts on a mount connected to the platform. All tall prints I make are perfectly straight, no wavy or wobbly vertical walls.