You can decide on the benefits based on the results of the Internal Equity Analysis and Job Classification (Job Grading).

The important thing is, work closely with the employees; HODs and engineers themselves because only they truly know and understand their work requirements.

Decide on the Internal Equity of the job positions first e.g. Engineer 1 & Engineer 2, Assistant Engineer & Senior Engineer, etc. You can use various models to do this, but one of the common one is years experience. This is the easiest one, but there are high risk of shortchanging competent engineers who have less years of experience. You can also do it based on competencies, but then you need to identify and create the competencies matrix for that. A hybrid approach is much more desireable, but you'll need to give good justification on the weighing of criterias.