You could change all the arms at the front or just the worn ones. If you change them all, that would then make poly bushing them now or later quite expensive. In my limited experience with 156s, I've found that lower wishbones can wear at any time, but if one top one is worn, then the other top one isn't far behind.
Balljoints cannot be obtained seperately- the whole arm would be changed.
I've never changed 156 bushes for poly ones, and have still yet to fit the poly buses to the 155 front wishbones- so I can't say what the differences would likely be. I've fitted poly bushes to a few 75s in my time, but think that a wise tyre choice makes a much larger difference to these cars.
If you are changing the anti roll bar end links, I take it there was some wear in them which was producing noise? Wear in the link bars does not affect suspension function until one of the links are so worn it seperates.
I generally think that if the rear suspension is in good order, the car will be stable. If the front suspension is good, that gives the car it's precision. I can't image what rear only poly bushes will do to the 156 would do. All the bushes in the rear lateral arms would have to be changed, as only the other bushes would have a very minor effect handling wise, so it depends exactly which bushes are available. I personally think that new rear suspension strut top mounting bushes would likely produce a greater effect on a car 100k+. It would certainly allow the dampers to be more reactive, and not only improve geometry consistency.
My personal thoughts would be to change both top wishbones now, along with one lower wishbone.
Change the rear strut top mounting bushes.
Change springs and dampers.
Allow the new suspension to settle a week and then have the geometry checked/adjusted.
Perhaps polybush the rear arms as a set at a later date when they become worn.
Hope you continue to enjoy your 156 for some time to come
BTW, I recently fitted a BMC air filter in the standard airbox, a Supersprint centre and a CSC rear silencer to a friend's V6- very nice. If you do the same, I promise you wont be dissapointed.