You should be able to use your wifi to download using app world. Having said that, if you are afraid of data being used accidentaly on your device even though you don't have a data plan and it costing you extra $, try calling your carrier and have them block cell network data to your device. This will eliminate any chance of data being unintentionally transferred through the cell network.
Another thing you could try is to turn off your mobile network, and only turn on your wifi in the manage connections options. This will mean any data is running solely through wifi.
Another thing you could try is to turn off your mobile network, and only turn on your wifi in the manage connections options. This will mean any data is running solely through wifi.