First let me say there are many ways to get to a flat surface for rail mounting.
I'm not following your use of the word "sub" here, probably a translation issue.
In any event once you consider welding anything you need to realize that welding distorts metal. How badly the distortion is depends on the materials being welded, the technique used to weld the metal and just how much welding actually takes place. In an ideal world the welded structure would be stepped relieved after welding which might be a problem if you don' have a local heat treat business.
If you do get the frame heat treated one of the best ways to flatten and set up the frame for rail mounting is to have the frame machined on a large mill. This can be expensive depending upon local conditions. However what you get is a flat surface with hopefully. registration edged and parallel and accurately placed mounting holes. This route is a good choice if you have a largish budget.
It should be noted here that you may not need to weld any mounting plates on your frame if you intend to use epoxy leveling. What you need to verify is that the frame members are thick enough to hold the the threaded holes that get drilled and tapped to hold the rails in place. Generally you want at least 1.25 to 1.5 times the screw diameter so material for a 5 mm screw would need to be about 6.25 to 7.5 mm thick. That might be a bit thicker than some might suggest but I'm including a bit of loss due to counter boring or chamfering the screw holes a bit to make sure deformed threads don't impact rail installation. Also structural steel is a bit soft so a little extra doesn't hurt.
Quick edit for something forgotten:
1. When considering material thicknesses for screw holding (in similar materials) one approach is to assure that you have 5 to 7 threads engaged. A screw with a 1mm pitch would need material 5 to 7 mm thick using this rule of thumb. I'd still add a little bit to allow for a counterbore or chamfering of the hole on the rail mounting side. I tend to prefer a little extra thickness myself to a material that is too thin.
If you are going to do a epoxy leveling approach realize that if everything works correctly the Epoxy self levels. More exactly the epoxy lays down matching the curvature of the earth, that is very flat for most of us. So in theory you don't need a level if you use epoxy leveling to produce your flat surface.
Do I need a laser level ? please explain. and if you have a video on this issue of how to exactly level it and then sub it would be great.
Regards
Johnansaro
However for machine setup and maintenance a good high precision level is a very important tool. So I would strongly suggest getting a high precision machinist level. This especially if you intend to move the machine at anytime after the epoxy leveling is done. The reality is if you move the machine you will induce twisting in the frame which can be corrected by leveling the machine as part fo setting it up in its new location.