About the equipment
What is the exact model and rating, what voltage and phase does it produce, what is the fuel capacity, and what is its recent service history? A rating on a quote is not the same as the unit that arrives.
About service
What is the guaranteed response time if it stops, does that apply at night and at weekends, and who do you call? Get a number, not a general assurance.
Ask whether a replacement unit is available and how quickly.
About the connection
Who performs the electrical connection, is an electrician included or your responsibility, and what cable and distribution comes with the unit? This is where most disputes start.
About the money
How is the rental period counted, what counts as overtime, what is the fuel policy, what does the damage waiver cover, and what happens if you keep it longer or return it early?