It's worth womewhere between what ever you're willing to pay for it, and what he wants for it.
Not being sarcastic, but that's the way it is. Expect both prices to go up if you start running into mud and want/need it NOW.
Series III is better than series II. Better flow control to keep from spinning the rear out.
A series II sytem (1 piece flow divider) will have 8 wheel bolts. A series III system (split flow divider) will have 10 wheel bolts.
The biggest difference between the valves is that the split flow divider will limit the maximum flow to each wheel to about 18gpm (36 gpm total to the rear axle).
http://www.jmgequipment.com/JMG Equipment Company Products.htm Ran across one of these on a used 2188. Its built like a brick out house, i called them to get info and they said the shipping weight is something like 2675 lbs. List price is $14750 plus $750 for 16x26 rims. If you get one let me know how it works...like i say really really built, gets rid of crappy center axle section that always breaks.