Design looks pretty good. I do have a suggestion or two that would help. Especially with a single screw for the gantry. Bring the back of the gantry down straight rather than angled to offset the router. Leave the front part like it is. This will spread the load on the linear rails for the X. It will both reduce racking and give you a sturdier Y and Z. This will mean either a shorter X or you will need a little longer X rails and screw as well as frame member. Use ball screws. Double nut preffered, but you can get by with a single nut with only little cost on accuracy and longevity.
Also consider a brace between the two endcaps by the screw on bottom. You don't want the screw to flex on the ends trying to move the substantial gantry.
It's difficult to determine what kind of speeds you will actually be able to pull off. A lot depends on machine design, attention to building accuracy as well as motor size and drives.
The speed you are reaching for is attainable with steppers. Especially if all you ever take is minimal cuts. Perhaps some of the other guys will suggest some motors or kits. Good luck with this. I look forward to your progress.