When you say "spot trailers" you mean to drop for loading/unloading or storage?
If so I'ev had success quoting based of our trailer rental provider. so if TIP will charge you $800.00/ month for a local cartage trailer + $0.065/km, I use those numbers with a small incentive for my time. IF its my own equipment, i'll take a bit off those numbers since my costs are lower to compete with say TIP etc etc. Repairs are always on a case by case basis, small scuffs and scratches are going to happen so its up to how you want to handle them, anything major and can be identified that it happened while in your customers possession, the repair costs is passed through to them. It gets grey if your also hauling the freight for them, but keep in mind those are always 2 different services you are providing. Treat them as such. I'ev had customers in the past like our trailer rates but not so much our hauling rates or vice versa. In my eyes It's a value add service to them if they ever need.