The batteries on a Prius shouldn't be too much of a concern, at least for purchasing most secondhand models. I think it was the Google Taxis (or equivalent) that accrued some ridiculous mileage in a few years, and the batteries still tested as good. Its more their disposal that is of concern for me.
And based on that, a replacement battery should really be a worthwhile investment given their longevity.
The engine itself also seems to be holding up to the very hard stop/start life (although starting and immediately revving is hardly healthy)... I've noted some Used Oil Analysis' at the Bob Is The Oil Guy forums with a couple hundred thousand miles, still looks reasonably good.
And based on that, a replacement battery should really be a worthwhile investment given their longevity.
The engine itself also seems to be holding up to the very hard stop/start life (although starting and immediately revving is hardly healthy)... I've noted some Used Oil Analysis' at the Bob Is The Oil Guy forums with a couple hundred thousand miles, still looks reasonably good.