The ender 5 is based on 2020 & 2040 extrusions which are 20x20mm and 20x40mm V slot. Important to use V slot as standard T slot extrusions are not the same dimensions. The build plate style is a cantilever with its support rails on only one end. This design is just fine for small light weight parts but not possible for the 20kg+ weight which is possible to print with this large scale. In this build the building plate will be completely stationary and be integrated into the lower frame with the XY frame being in motion along the z axis. This has several advantages like all motors and electronics can be attached to the moving frame allowing for short wiring to motors and reduced electrical noise.
Source the applicable length for your build in this case the Y axis is going to use 2x 2040 extrusions because of the long length the weight of the X arm traveling the center would potentially bend but more on that fix later. The X axis will retain the stock size extrusion but could be made to any length also. This allows for only 2 extrusions needed for the upper gantry.
Z axis frame
You will also need 4x extrusions for the z axis which currently is using 2x 10mm linear shaft. You will be replacing those with the 4x 2040 extrusions with the same gantry style plates as the stock Y axis
Using Gantry plates is optional and can be replaced with linear shafts/ rails depending on your requirement. The 2040 extrusions have enough accuracy for this build and the gantry plates can be found from several sources for very little cost in comparison to industrial linear rails.
Z axis motion
Z motion normally is done with a lead screw which has many advantages and disadvantages. To start with the major disadvantage the stepper motor is holding all the build weight axially which will reduce the service life and be ruined unless you change the internal bearings to a thrust style bearing. This is not recommended as some motors can be damaged during disassembly. The alternative is to use a belt driven Z axis. The major issue is if power loss occurred the motor will give way and potentially break something as the gantry comes crashing down. This can be solved by using a planetary gear set which mounts to the stepper motor. This has a larger holding force because the planetary gears are setup to be torque amplifiers meaning a single turn on the input shaft will turn less times then the output shaft. With a high enough ratio the internal magnetism of the stepper can hold without power. Additionally this rotates the force transaxial and into the planetary gear set so the motor will never see axial force and should have great service life. The biggest con to this setup is belt selection. Cheap belts without fiber or steel reinforcement will stretch over time and have a slightly lower service life. Luckily belts are very inexpensive and are easy to replace which is why this style of motion system has been selected.
Many build surfaces can be used to fit the material needs. In this case the best option is granite because it is stiff, heavy weight, handles moderately high temps without any expansion or bending, and can be sourced locally in every major city at a decent price if a remnant can be used. Because of the mass and thermal conductivity of granite the preheating process will be extended but the trade-off is if power loss occurred the heat will last for longer and allows for more time to save a build if a failure occurred.
The enclosure will be mounted to the frame adding stiffness and thermal insulation value. The load acting on the exterior is relatively low but needs to account for the XY motion during printing. The exterior can be almost any sheet of building material even wood has an ignition point well above the environment temperature. The simplest and cost effective option is reflective XPS foam and hardboard sourced locally.
Because abs releases fumes which are not great for your health a carbon filter system will be added.
The electronics that are needed. A more capable controller board, touch probe for mesh leveling, 2 stepper motors with planetary gearing, and a dehydrator for environmental control. Potentially more items are needed like additional fans for circulating air within the chamber and maybe some upgrades to existing components.