Actually, a 3 point hitch backhoe on small Kubota will probably also void the warranty if you break something like the case casting. Far as used backhoes go, there are lots of them out there. My dealer seems to sell a bunch and most spend their lives sitting unused in a corner. he always tries to talk customers out of them but hey, the customer is always right so they buy them and they sit.
In my view a tractor mounted backhoe on a smaller Kubota (or any smaller brand for that matter, is the most worthless investment you can make.
I much prefer (when I need a hoe) is to go down to Black Swamp or Herc rentals and rent a real machine, I can rent a Case all wheel drive extenda-hoe with my choice of buckets for 250 bucks a day, fuel and delivery included. Don't have to grease it, change the oil or even fuel it. Just use it.