Reply to digger:
I have a 22 b we have been working on and have not had much trouble finding parts althouth some are pricey like 190 dollars for the cap and rotor for the pony start on a 318 d .I don't know anything about a 15 b to compare the two for you.Their is a place in cleveland ohio called acr that has parts on the shelf that we have needed for ours and not to bad of prices.If you need a phone number post me back and I will find you one.
I have both a 15B and 22B. Both of course have their disadvantages and advantages.
My 15B has a shovel front and lattice boom. I have up to 85' of boom for it, but you can't hardly lift a wet roll of toilet paper with a shallow angle on the 85' of boom. I run it with 40' as a dragline and that seems to work ok. It is a 1/2 yard machine, it is around 8 feet wide and weighs about 35,000 or so so hauling isn't a big issue. It also has a 453 in it, so no pony motor to start..
My 22B has a D318 cat in it. I think they put it in there to act partially as a counterweight, as it has far more power than the machine needs.. I have 55' of boom on the 22B. It is s 3/4 yard machine, but typically guys would put 1 yard buckets on them. Mine weighs in at 56,000 and is just under 11 feet wide, so it needs permits and pilot cars to go down the road intact. As a crane and a dragline it will lift much more and reach further..
There are many many 22B's around, I have also found a few 15B's but the 22B was a common workhorse around here. You can still buy a new one if you want to import it from England (or at least you could a few years ago)
I use my 15B as a lifting crane for working on most of my projects as it is smaller and easier to move around the yard and other machinery. The 22B only plays dragline and that isn't very often. Or it will be called in to lift the really heavy stuff. I don't have my load charts handy but I think a 22B set up with the right udercarriage and counterweights will lift more than double a 15B.
Now that I have my excavator working the 15B is going to get it's shovel mounted and become a showpiece. The 22B is beat up enough that one day it will probably just get scrapped or dissappear into the blackberries.. but then I am also working to buy 2 more 22B's with all the attachments..
You can contact me privately or here if I can help you out with any other information.