i am also looking to build a printer with 600mm printbed

 

20x20 extrusion worries me for a printer this size. I did a lot of stress analysis and positional error analysis due to bending under acceleration. For an X=450 Y=600 printer, I had to go to 30x30 extrusion for the X axis gantry and 40x40 for the whole frame including Y guides. If you intend to use 20x20 on a 600 wide X gantry, an acceleration of just 10000mm/s^2 will give you around 0.8mm of deflection which is a significant error on a 3D print. If you don't mind sacrificing speed for size, then it's an economic solution. Another problem I had was finding a bed heater for a large custom sized build plate. I eventually found Standard Keenovo Silicone Heaters who seem to know what they are doing. Don't underestimate the power required for heating a big build plate either. It will be over 1500W for a 600 x 600 bed!.The supports and thickness of the plate also become important over such a large size. I would recommend 8mm or preferably 10mm, and using one of the pre-ground cast Ally plates makes life easier. Good luck!