Good find. Now that's the way to buy a Banshee. Even if it's not set up quite how you want it, sell some aftermarket goodies and use the money to buy what you want.
As far as weight difference, a few things to consider are that '87 didn't have footbaskets, so that's probably 5 lbs. At some point they changed the swingarm carrier bolt design to 2 long bolts with spacer tubes, instead of 4 short bolts, so that's probably 1 pound. So, for the actual J arm vs A arm suspension, it's probably only about 3 lbs difference. Maybe less, considering that the early brake calipers with the big cast iron brackets are probably heavier than the later ones.