The 2nd remote hydraulics was optional on a Series IV. You have split the valve stack under right side control console and add another spool to get the 2nd remote. I priced one from Cook Tractor in Clinton, MO in 2006 and I think it was $350 for the valve, lines and long bolts. No sure if that included the control lever/linkage. Quite a bit of stuff to take off to get at it to do the work. The valve stacks are the same on a 170/175/180/185 so those hydraulic parts are not too hard to find used from the salvage yards. Seal kits and some other small internal parts are also still available from AGCO.
As far as manuals go, JMS is right on with the original AC operator's manual and an IT shop manual. I also have a parts manual and I find those to be extremely helpful, sometimes more than a shop manual. Don't buy the current D17 shop manual from AGCO, it's a kludged together edit of the shop manuals for all four series and they left out almost all the specific stuff for the extensive changes in the Series IV. It's a waste of money. If you can find an original ALLIS shop manual printed back in the 60's, that would be great but they are hard to find.
Good luck with your Series IV, probably the best utility tractor Allis ever made. I have one too. Mike