Question

Why should the unit charge be positive while measuring electric field strength?

Answer 1

It's nothing more than a convention, but one possible answer is that \[W=q\int\vec E\cdot d\vec s \implies W>0\iff q>0\]when \(\vec E\) and \(d\vec s\) point in the same direction, which is more intuitive than having an electric field moving a positive point charge in the same direction of the field do negative work.

Answer 2

Yes, convention is that E points in the direction it would move a positive charge.