Thanks Silyavski!

I will go with HTD belts then.

Ballscrew standard machining leaves 15mm space for the pulley. Standard pulley is about 26mm. How do you solve this?

I think Silyavskis steel Z axis is coolest one I have seen :) I have studied that design but dont feel certain that I can pull it off. As I have good access to 20-25mm aluminium flatbars I go with the "usual" aluminium Z axis design.
Btw what is the weight of you steel Z axis, Silyavski?

I will beef up the frame and will rethink the bed fixing system.

I need to order rails and screws before EURO is totally useless :(