Hi Dave;
Can't speak for the other models, but my B7200HST has been one of the toughest machines I have ever owned. While overheating is possible, it mandates near abuse of the machine. I purchased my B7200HST new in 1983, and other than replacing a split pin in the HST rocker pedal shaft, have done nothing other than scheduled maintenance. Pound for pound, I firmly believe my B7200 has more transmittable power than my old L3350 or my present 1998 L5450. I passionately refer to the 7200 as the "tick"... sticks to everything, tenacious as all getout. I'd buy another in a minute, and sincerely feel the more modern B series are not nearly as robust as the older series.
btw, yes power steering was offered on both the B7200 as well as the B8200. I don't have it, but then again i don't have a loader on mine. For normal use (ie. perhaps loader, but not an extreme heavy load in bucket), power steering is not necessary. When I was initially shopping, i tried 7200's and 8200's side by side... PS and w/o PS. Difference was minimal to non-existent. The salesman (who i still deal with to this day) stated it was a poor value, and instead pushed the 4WD which i virtually could not do without. I often mow (RC60-72H 60" midmount) on severe sideslopes and the 4WD allows true contour tracking and confidence. My 7200 with turf tires will run parallel on 2.5 to 1 side slopes and has never in 22 years given the slightest hint of instablity. Can't say that with my bigger machines.
If you need specs and or literature on any of the tractors (other than the B20) listed, i have the orginal literature plus full Kubota shop manual on the 7200HST. Like some of the previous posts, I think all these little older Kub's are near bulletproof. Can't say that for my 1968 Case 580CK, or Ford 9N, or a bunch of other older stuff that have somewhat undeserved reputations for longevity.
Good luck in your pursuit!