You don’t need any of the sensors but speed sensor is worth having if you want a more consistent speed reading when riding.. Even when gps signal is strong, GPS measures speed based on travel time across a longer distance than a sensor on your wheel so you will get more consistant reading from the sensor. I find GPS gives more accurate data on distance and elevation than a sensor alone so the tw complement eachother. The other sensors will be useful for you for training and monitoring level of effort during the rides.

The wahoo computer pairs with an app that has some route planning capability. But it also syncs directly with ridewithgps, which has superior route planning capability for free via its basic membership, including turn by turn, so thats what I use.