Change brushes every 5 - 600 hours. Waste tank and dump valve seals every 12 mth, Check for leaks etc, obviously PAT test every 12 months. Carry spares so if it breaks you can fix it quick! I have 2 lamb 3 stage vacs, 135 psi shureflow pump, fuses, quick connects, and solution pipes for hand tools as spares just in case! Worst case scenario only out iof action for an hour or so if it breaks down. Bit of an outlay to do this but could cost you more in work ( especially this time of year ) if you have to wait a day or two for parts.
Geoff