Im guessing the A/C condenser is sitting on the counterweight ?
keep this cleaned out..........wash it out daily when its in constant use.... after washing make sure it's dried out before you fire it back up
same goes for the evaporator if its easy access...only you dont need to dry it out after cleaning
also have a spare A/C compressor drive belt on the shelf
Right then, previous experiance of these machines i would recommend the flowing parts.
Water pump and gasket (bearings fail due to dust build up on pulley).
Starter motor and relay, bottom mast anchor rose bearings, 'o' rings for the two feed pipes into the squash plate housing.
(these are special high preasure rings)
Horn push, Key switch and hoses (you will need the mast number when ordering).
Service filters (donot clean or change the air filter until the indicator says so)
Providing you grease and carry out regular maintance these machines should give you good service.
Are they the X model ie has a diesel vw engine or the perkins engine?