The learning curve is very short, especially if you’re already familiar with operating equipment (in terms of being careful, respecting the power of hydraulics, etc).
I actually bought a used large one (Hitach EX-60UR) (about 6.5 tons weight) around 2008, used it for about ten years, then sold it for pretty much what I paid for it.
Depending on how large your stumps are, you generally will want the largest excavator that you can afford. Makes it easier if there is a thumb on the bucket, especially if there are large rocks around the stumps to deal with.
Just a suggestion - look on Craigslist or somewhere for someone who does that type if work. Might be less expensive and less of a headache to just hire it out if this just a one time thing for you.