Ask for the load analysis, not just the system size. A proposal that starts from your bills and your daytime consumption is worth more than one that starts from how many panels fit on the roof.
Ask what happens in an outage. Grid-tied solar without batteries switches off with the grid. If resilience is the reason you are buying, the design must say which loads are protected and for how long.
Ask who installs and under what licence. Solar PV design, installation and sales in Kenya require EPRA licensing. Ask to see it.
Ask about the roof. Structural capacity, waterproofing and access for cleaning decide how the system performs in year five, not year one.
Ask how you will see the output. Monitoring is cheap, and a system you cannot see is a system you cannot maintain.
Ask for the payback with your own numbers. If the vendor cannot show the calculation, the number is a guess.