Thanks guys for your inputs ...

In the meanwhile a lot has changed ...

When I realized that I'm gonna spend almost same amount of money on this machine and on machine 1500x750x300(effective travel will probably be around 1300x500x180) I switched to bigger size right away.

So I changed drivers and went for this - THB6064 special 1.1 description - around 45V, and 4.2 A.

Also I bought some aluminum to see and test ... It seems to me that 15x150mm should be stiff enough in combination with 40x40 and 40x80 aluminum extrusion. Extrusion I will use as "support" on x-axis (1500mm - I plan to use 40x80 extrusion) and y-axis(750 I plan using 40x40). On them I will mount rails.

I will also use SBR16 rails for all 3 axis, I'm thinking that this is also adding rigidity.

I will use Ross77 advice for z-axis cause it makes most sense to me also, also I'm thinking using 15mm thick aluminum for z-axis plate.

y-axis I will also design according to Ross77 comments putting rails vertically and opposite, lower facing down and upper facing up.

So gantry will be in 15x150 mm aluminum and 40x40 combined with 40x80 aluminium extrusion.

I'm guessing that this should prove to be stiff enough.

Also all ballscrews I'm gonna use are 1605 together with BK12/BF12 end support.