This is my first BUILD please help.

[DigiSoft]

Hi. I am researching this field for months and I decided to make one. I ordered almost all parts except the linear rails. I am going to make them.
I have all the parts but I'll post pictures tomorrow because it is late now to take pictures . Anyway this is the parts info:

1. 4 x Axis Driver 2M542 4.2A & Breakout interface board for Router system for CNC
2. 2 x ballscrew RM1605-1300mm (for dual X axis)
   
3. 1 x ballscrew RM1605-900mm (for Y axis)
   
4. 1 x ballscrew RM1605-250mm (for Z axis)
   
5. 4 x flexible couplers
   
6. 3 x NEMA23 425 oz-in CNC stepper motor /3.0A (two for X, one for Y)
   
7. 1 x NEMA23 287 oz-in CNC stepper motor (for Z)
   
8. 4 set BK12/BF12 ballscrew end support
9. 12 x SBR20UU BLOCKS
10. 3 x Cable drag chain wire carrier 10*20mm 1000mm (40")

I have all of this parts and I hope I am on the right track, I'll be so frustrated if I made some mistake because my budget IS VERY VERYYYYYY TIGHT.
Anyway I would like a little help.

1. What is the best alu plate thickness I should use for the Z construction and the gantry. I was thinking of using 20mm plates. Is that good?
2. How should I place the linear support rails on the Y axis? Should they be mounted on the front of the alu plate or one at the top and one at the bottom? What is better system?

I have some drawings on paper but still nothing on PC and I am not sure if I make because I am not that good in 3D software.
Please help me with this first questions. THANK YOU

PS: Like I said I'll post pictures tomorrow.