z-axis - lead screw mount critique

Gents,

yet another question for all you experienced peoples, here's a z-axis assembly design:

Attachment 8039

Two questions:

1) anything wrong with the above? I've used a double row angular contact (10mm ID) as the locating bearing and a standard deep groove 10mm ID on the bottom purely for side stability. Two lead blocks 35mm apart with a spring in-between (not shown).

2) I was planning to go for a poor mans leadscrew - threaded rod, but I'm a bit worried about getting something straight. The screw is not long, around 300mm. The 'carrier' (the whole assembly which moves up / down) will be 4kg, 'cutting' forces, both axial and radial shouldn't exceed 100N. I was thinking of upgrading to a ball screw, but costs grow rather fast with such approach (single ball nut is roughly £50), would a trapezoidal screw do the trick? I'd prefer a precision / fine thread, which seems to be tricky with trapezoidal screws. Standard M12 will give me 1.75mm pitch or 1.5 / 1.25 with fine pitch, TR12 is 3mm pitch.

Main thing is that the movement off the 'carrier' is limited to roughly 5mm. Yes that is correct, 5mm. It needs to be super solid when held in a position, so zero axial play and no backlash.

Thanks in advance for any input.

Regards,
dsc.