My son inherited a 320 wagon with a host of problems, most recently a vibration from under the hood that sounds like a tensioner yet his photo of the problem pulley appears to be something else. Can anyone identify the problem and offer a solution?

 

that is the belt tensioner and it is the original one. the new ones are updated. replace it. or it will break off and leave you stranded. when they go bad they make kind of a clicking, clacking sound which you usually will hear at idle.
by the looks of it is is already busted.

 

Let me send you another photo which I just got my son to take. The tensioner pulley appears to be the one at the top of the belt, however the pulley that wobbles is the one on the right. Is that an idler pulley, I can't see from the photo that this pulley goes to an accessory.