Rental bicycles should be robust and easy to fix. Therefore it is best to choose bikes and components of good quality and that require less maintenance. Bad or cheap quality bikes are usually not convenient at all in the long-run, also considering the reselling price.
A unisex frame together with quick release adjustable seats is a must for rental bikes because riders comes in all shapes, sizes, age and gender. The comfort should be assured to all of the riders.
We encourage the purchase of bicycles with few gears - 3 gears is what we recommend, up to 7 if the location is quite hilly. Regardless the number of gears, it is better to buy bikes with inner hubs which reduce the exposure to dust, salts,water etc.
If suitable for your city, pedal brakes are a good solution and requires less maintenance than rim brakes. Alternatively, roller brakes are more expensive but require even less maintenance.
To secure the components on the bikes you can purchase special nuts (security bolts) with irregular holes that can be unscrewed only with a special adapter. Regarding the saddle, purchase the bikes that have built in anti-theft protection, or find good alternative solution.
We recommend bicycles with lights built-in or alternatively, magnetic lights as they do not require much maintenance. Remember to look into your local traffic regulations about the accessories such as helmets and lights.