Prices for these old tractors are so regional it's hard to give any sound advice on their value, I paid $4K for mine with a 7' MF BB 12 years ago, which for the market in my area at the time was a good price. Mine's a gas 135 Special, basic gage package and single stage clutch, no power assist steering.
Things to look for aside from checking the basic functions of the tractor include;
1. there's a drain hole in the lower casing of the tractor just aft of the block, there may or may not still be a cotter pin sticking out from it (meant to free the hole of dirt and other possible obstructions), check it to see if there's any oil coming from it, if there is it could mean the rear crank seal is bad.
2. three point for operation, a lot of these old tractors sit outside, if the shift lever boots or seal for the draft control spring are bad they will allow water to ingess into the case and contaminate the oil, a sand colored oil on the dipstick will give some indication whether or not this has occurred.
3. the water pump pulley on some models is fairly large and heavy and is a press fit onto the pump shaft, grab the fan to see if there's any radial play (can't say the 135s are "bad" for this, just something to be aware of). Not expensive to fix but something that will need to be addressed.
Mine's a gas model so I can't give you anything to check on the diesels. Things that I ended up repairing / replacing on mine were, for the most part, typical wear items on a tractor of this age.
Good luck in your search, they are a workhorse.