I don't see why the backhoe feet could not be put down on on anything you like... I've seen old tires used for the purpose. I've got pads on one hoe and the original steel feet on the other and both tend to sink too much..
The first time anyone uses a small backhoe to dig out stumps is likely to be an education. Take it easy on the hoe, it is easy to stress the backhoe too much when digging stumps. Expect to dig twice as deep as you thought it would take, twice as wide, all the way around, and still have to get off the hoe to chop & cut some roots.
On the box blade, for such a simple implement, they do come in quite a range of different quality. The heavier the better - always. For that price it should be noticibly more heavily built than a less expensive one, reinforced. Some will have a flip down back blade for pushing as well as pulling.
New or used doesn't matter much on box blades. What does matter is weight and strength.
rScotty