Not sure if you'll have any advice on this, but I'm helping my neighbor change his front lawn into a native plant garden, and planning to include a little water feature with a pump running off a solar panel. I've barely begun researching any of this part (been lugging dirt and cinderblocks from our local Everything is Free page), but I always try for used stuff first. If you have any recommendations I'd love to hear them.
if the water feature is really tiny, then the pump and panel taken out of one of those decorative countertop fountains – but that isn’t going to be able to power much more than a soothing trickle …
next stage up would probably be something like pool cover pumps and pool drain pumps – but don’t know if those would be any cheaper than proper pond pumps that are designed to be run unmaintained for long periods of time
For the pump, maybe you could look into (secondhand) aquarium pumps? They're meant to be constantly running. Flow rate is fixed though, so you'll have to decide the size if your pond first.