I have rebuilt over 10 of these from a small engine business that was closing. I would replace the drive belt, and also the bushings that the transmission axles go thru. They are easy to find with the online diagram. As for the hydro transmission goes, drain all the old fluid out and replace only with "Honda" fluid. No substitutes. Make sure to fill it up where it overflows, then run it with a drill and fill it again so it overflows. Then put the top cover plate back on. The rubber expansion bladder will expand if needed but if underfilled it will make buzzing noise when enguaged. You can only service these on a bench otherwise there is no way to fill them properly.
Also, when you install all the height adjusters and the metal sleeves into the plastic hole, coat them with a non petrolium wool wax or fluid film so they do not rust / swell up. In a non abuse situation, these will rust, swell up, and crack the deck in a way that makes you think it was abused.
While the engine is off, remove the valve cover and slide the cam pin out, and replace the o ring with a brand new one. Slot will face towards front and the valve cover holds it in. Also make sure your valve cover has the little edge inside the keep the cam belt on. If it is the early style, get a replacement valve cover. If this ever starts to leak, the pin will not come out with the enigne installed, and it is not pleasant plucking off the oring when the pin is half in and half out..
I think it was worth the $400 as there is nothing built that feels as comfortable as these to operate.