Some of the list depends on your setup. I crawled around the van and made a list of every single component down to jubilee clips and electrical connectors and bought at least one of every single item. When we have a van kitted out for the franchisees, the order includes a spare of every single component used (bar obvious stuff like tanks!)
Even then things have broken that I didn't know about or hadn't noticed. When that happens I buy two replacements and one goes in the spares box. Also, every time a part out of the spares box is used, I reorder.
You also need a full toolkit. No point having a spare if you can't fit it. There are plenty of good, surprisingly cheap tool kits online. Not good enough for a professional tradesman but enough for our volume of repairs.
The thing that saves me and the franchisees more often than anything we carry is that we have a two-man setup in each van. If there's a fault in one they can carry on with the other reel, pump and controller pending repair. Cost over ten years? About 10p a day to ensure you never have to give up and go home.
Vin