Question

What should be the minimum initial velocity with which a missile has to be launched so that it reaches a target of 1000km away in the same plane

Answer 1

Nuts to these made up hypothetical, wanna be physics problems.

Answer 2

maximum range is when the angle is 45°

so, minimum velocity need for a given range is at 45°

Now just use your range formula using 45° to see what initial velocity has that range. That is your minimum required velocity.

Technically, a 1000km range has to include adjustments for the curvature of the earth, since it is so far.