The thing to remember about the older (say 15-25 year old) 70HP tractors is that it will probably be very similar in size to a 80-90HP tractor. The engine, transmission, wheels, tires, frame just about everything may be close enough in size that you might as well open yourself search up to 85HP and enjoy the extra HP. Price may not be much different, either.
For a larger example of what I mean: My Case-IH Magnum 270 is basically the exact same tractor all the way from the Case-IH Magnum 180 up to the 270. The only serious difference is the engine HP and possibly pump size, clutch packs, etc. Side by side, they are very similar in physical appearance and dimensions.
Remember that when tractor shopping. You may be wanting a 70HP tractor, but find an 85HP version that, side by side, looks identical to the 70, but you get more power in that same size package.
For any of you looking at used tractors in the 75HP range with limited funds, take off the orange glasses and look at a Case-IH Maxxum and McCormicks. Serious, albeit smaller farm tractors with heavy built features. They have engines and transmissions that can go in larger units, so you know they are HD.