The PM400 was a good machine and will more than likely do the job for you. This is what I would do before I would even think about shelling out $8,000.
1) Check all fluid levels and the smell of the oil to see if it has been overheated (burnt smell)
2) Chech all linkages for smooth tight operation.
3) Check that PTO engages smoothly without any noise.
4) See if tires have cracks (checks) from age.
5) Check out the radiator fins & inside. Look for corrosion in the cooling tubes.
6) See how old the battery is.
7) Check the deck blades for levelness to each other. Could indicate bent spindle or deck.
8) Run the engine and check for smooth operation.
Another suggestion is to purchase a new Gravely PM272 w/25hp Kawasaki. Price my be slightly more than $8,000 however you will be getting a new machine with a two year warranty and you will be able to cut your grass in much less time than you could with the out front machines. Go visit your local dealer and get the whole scoop.
Mission: Constant Never-ending Improvement