They are good units. Don't know what to tell you about the hour meter, you're just gonna have to take that one on the word of the owner. Not sure why it would smoke under idle. Maybe after you first start it up, but after she is a little warm, it shouldn't have much low RPM smoke. Likely not injectors, would more than likely be valves or rings if she is pulling oil into the motor.
The 3PH only goes down with weight, it isn't hydraulic down, only hydraulic up. You may want to press the lever down, then stand on the arms to see if they go down. Then you can see if the the hydraulics raise them back up. There is a control valve under the seat that sets how fast the arms will go down, you may also want to check that valve.
Be careful on the maintenance stuff. If you can talk the owner into draining the tranny fluid and checking the hydraulic filter screen, that would be ideal. That will tell you somewhat of a history on what kind of maintenance it has had. If that screen is all plugged up and nasty, you may want to pass on the tractor. The screen is located on the RH side of the tractor, below and slightly to the rear of the break pedals. You can search the site here for pictures on exactly what it looks like.
Good luck, let us know what you find.