For the balloon, mass is a critical factor.
The lift provided by the balloon is a function of several parameters, such as the diameter of the balloon, type of lift gas used, and density of outside air.
Once the lift is computed, the mass of the balloon and assorted hardware, such as cables, must be subtracted from the gross lift amount to determine the net mass that can be kept aloft. This net amount is the total budget for the balloon payload.
The mass of the payload must include the structural elements as well as the electronics, sensors, and batteries needed for the payload to function during the defined mission period.
Here are a couple of examples of DIY payloads similar to what the Mars balloon will include: