Hobby Machine Recommendations £1000-£1500

[CodyPilla]

For a 400x400 machine, 1605 screws would be good enough.
They do limit top speed compared with 10mm pitch screws, but at 400mm max travel distance, that's not really a major issue.

As for rail size, 20 size is actually how to get rid of gyno overkill for most machines, but the larger mounting screws let you get away with simply bolting them down, and the higher height often helps with component positioning.
In terms of actual forces the rails need to handle, you could probably drop to 10mm rails and still have a perfectly good machine, but they'd need to be mounted as per the official recommendations, with the rails clamped into position against a reference edge, and not simply bolted down.

Is it necessary to get official recommendation first?