Implement buffer. The buffer can be used every time.
Buffer is mandatory if the user selected a point geometry from gazetteer.
As soon as the user insert a number (keyup event, with a timer to avoid too much requests), the WPS buffer request starts and the buffered geometry is added to the map.
The download request includes the buffered geometry.
Check for tag in the capabilities to know if the layer is a single layer or a group.
Trying to download from a group layer will result in a failure.
At first sight, there is no way (with the current OpenLayers implementation) to distinguish between a single Layer with an empty Keyword list ( tag) and a Layergroup (no tag at all)