It would have be nice to know if you want to use them in wastewater or in drinking water. It make a difference when selecting a unit. I will be general in my answer since I don't know andf will gives you some criteria for both type of water.
Many criteria should be considered but to name few you should take into acount the characteristic (suspended solids, turbidity, iron, manganese , color , etc, depending if you are in wastewater or drinking water) of your water and minimum, maximum and peak flow. Bacterial criteria that you want to reach is also an important point.
I would ask question about:
-Similar project done by the supplier reference
-Cost of maintenance and ease (spare parts and time required), does it require special equipment and if so is it included?
-Life duration of main components
-Security aspect of the installation (No UV light visible from floor, no risk of falling)
-What control device are included
-Agreement of components with the local regulation (ex: UL, CSA, NSF, etc.)
-Is the module can be remove without stopping the flow, does it need a back up module while doing maintenance?
-If it is for drinking water does the transmittance is equal or better to 85%
-Criteria reduction that you want (how many log) to meet your local regulation
-If in drinking water it would be better to consider redundancy
You can read the ten state standards on UV systems it may help you too.