I operated a 318 for a man when I was in college. This experience prompted me to buy one when I first bought my acreage in 2003. They were very well built machines for their time and I think the last of them were 1992 models. They were powered by Onan air cooled engines, which were supposed to be some of the better quality air cooled engines in their day. I had just over 1500 hours on the tractor and engine when I traded it for a newer model. When you have to start working on the Onan engines, the parts tend to be rather expensive. I replaced the starter on mine and it was close to $200 from the local John Deere dealer. I believe the best width cutting decks are either the 38 inch or the 52 inch, primarily because they are "deep" and I feel the metal may be a bit heavier guage steel, whereas the 46 inch deck is shallower and always seemed "flimsier" by comparison. The deep decks seem to handle taller and wetter grass versus the shallower deck. My one complaint on the mower deck is the minor "stripping" that can be seen in the area where the blades overlap. In taller and tougher cutting conditions, especially once the ends of the blades are knocked off, the stripping will be more prevalent. I DID like the single lever hydrostatic control over the foot pedal control, but fortunately my newer model has cruise control for setting the ground speed. My family and I also liked how nimble the 318 was with the power steering featured as standard equipment on that model. It was so very easy to operate.
If maintained, just like anything else, these machines are good for a long time of service because they were built well and were the heavy duty garden tractors of the John Deere lineup when they were built. The two "weaknesses" of the 318 I experienced involved the starting system and the fuel system. With respect to the electrical system, as the cables connected to the starter and other connections aged or started to weaken in connectivity, there was a problem with getting a full 12 volts to the starter, which resulted in the starter solenoid clicking rather than engaging. Deere came out with a "relay" kit to help ensure the voltage at the solenoid and starter was adequate to engage the solenoid and starter. I discovered this problem and resolved it by simply putting a battery charger on the tractor's battery which slightly bumped up the voltage and all would work. I found out about this "update kit" about the time I decided to trade up. The other issue involved fuel draining back to the fuel tank. I found out about this on my former employer's machine, which was resolved with a squeeze bulb primer in the fuel line. Apparently the fuel lines, as they age and harden, will start to allow a tiny amount of air to bleed into the system allowing the fuel to drain back from the carburetor. I replaced the fuel lines on mine before this happened.
The engine is likely to wear out before the rest of the tractor and drive train are in need of service, in my opinion. I had originally planned on re-powering my 318 at the time I owned it. If I recall, there is a man in Hastings, Minnesota, who sells re-power kits for all makes and models of lawn and garden tractors. I think he typically sells Briggs and Stratton Vanguard engines for most of the re-power kits, but may also dabble in Honda engines, too. When I was getting close to doing this, I found a John Deere 425 all wheel steer with low hours and a 54 inch cutting deck. I had also tried out a 425 with the same mower deck. I was impressed by the better cutting job and I liked the water cooled engine made by Kawasaki. If I had not made that move, I would have re-powered my 318 and probably still had it. I was able to upgrade to this 425 all wheel steer (with just under 400 hours) for within $200 of buying the re-power kit, and I would have had to either done the work myself or hired somebody to do the work for me. I opted to trade to a newer and lower hour machine........................which I have had for 10 years and really do like! Bottom line is the 318 was a great machine in its day, while the 425 was an improvement over the 318 when it came out. No machine is perfect, and one if likely to find some weak points on all, but if you are aware of these weak points, one can probably head off problems with some preventative repairs or maintenance, too.