Insurance - i'd want it to be completely comprehensive.
Experience with dogs - how many dogs have you had, what breeds, what ages, any behavioural issues you have experience in managing.
Details. How many dogs would you walk at any one time? How long would any dog be in a crate in the back of your van for? How, if you are walking multiple dogs, from multiple households, would you manage possibly personality clashes? Would you walk on lead or off-lead? Where would you walk? How long would you walk for if I paid for an hour? How would you decide which dogs you took on to walk? How would you build that relationship with the dogs before the first walk?
I'd want to see the vehicle my dogs would be transported it. Be confident my dogs wouldn't be shoved in a crate with other dogs. I would want to know my dog would never be left unattended in your vehicle etc.
Any relevant qualifications (which honestly, I think are garbage but many people do care about that).
Policy for last minute cancellations due to extenuating personal circumstances. Policy for cancellations because you are unable to make it (full refund, alternative slot etc).
Honestly though, experience and how many dogs at any one time is key. Personally, even though I don't use a dog walker, I would never ever use someone who walked dogs from multiple households together and who had never owned a dog and had no experience in dog ownership/handling - it's just a recipe for disaster.