Lefteris,
Getting the layer to display in the attributetable is probably the easiest part of your whole desired workflow. Are you planning on hardcoding the layers that will get buffered or just have them defined in a XML configuration file? There is not a widget out there right now that does your exact workflow as you probably know. The Point Buffer Widget might be a good starting point for your development if you are looking at allowing the user to dynamically draw on the map and have that buffered (of course it is only designed for points right now but its a good start). I guess a more detailed workflow would be needed before I can give more direction.