I have a 1958 Series 1 Gas. It is a great tractor. The power director gives you "live" PTO and hydraulics, plus a 2- range transmission.
THe series 1 through 3 bring from 2000 to 4000, depending on the shape that it is in.
I have heard of only one problem - if it has been abused, it will jump out of 3rd gear, which is expensive to fix.
Do a compression check on the engine, when brand new it would have been 140 psi on all cylinders.
THe early series tractors had limited hydraulics, usually just the lift, and one single acting remote outlet. It can be upgraded to two-way with the addition of a selector valve and a return line back to the sump.
THe hydraulics on the Series 1-3 are low-flow (4gpm) and high pressure (3500psi), so you have to be careful about the cylinders that you use.