Ian,
If the tractor runs at all, you should be able to make basic checks on the workings of the electrics, the engine, clutch, transmission, and mower deck pretty easily. Some serious red flags are if the transmission pops out of gear, knocking noises from the engine, lots of smoke, etc.
Don't expect a perfect tractor, but assuming it runs, you can gauge how much work needs to be done by getting a feel for the controls, how things sound, etc. Does it look all monkey rigged? Does it look like it's sat outside for ages? How much play is in the steering, how smooth is the clutch, how does the mower deck look and sound, even the condition of the tires come into play.
If the tractor doesn't run, try and figure out why. If the engine is "stuck", it could be as easy as squirting a little oil into the cylinder and working things free, or as serious as a seized piston or rod journal. If the engine spins too freely, it could be as easy as a blown head gasket, or as serious as a broken rod. If it seems to have compression, it is probably an ignition or carburetor problem, which are usually fairly easy to fix. Checking the spark plug fouling can clue you in to the engine's condition as well. Try to spin the engine by hand to gauge an engine's condition if you don't have a working starter. It might be a good idea to bring along a charged lawn and garden battery "just in case" the owner claims the battery is dead.
If it doesn't run, you can still try to gauge the condition of the rest of tractor by doing basic checks like checking the steering, seeing how crisply it goes into gear, examining the mower deck for rust and spinning the blades by hand to check the condition of the bearings. It won't tell you as much as a running test, but it will uncover obvious problems.
With the exception of transmissions and possibly brakes, there isn't a whole lot of difference between a 100 and a 149 in the way things work. The engine, electrics, PTO and steering are very similar, and there are really only minor differences in the mower supports and mower decks.