You are getting good advice above from some very smart pond people.

For the first 40 years, did the pond fill completely after big rains, and even run some water through an overflow pipe? Can you look at the lay of the land and determine if it does fill with surface run off water? (It could still be a groundwater pond that was placed in low ground. However, that type of pond should react a little differently to heavy rains.)

If you can get your pond full after a big rain, put a water gauge ruler (marked PVC pipe, etc.) into the pond and mark the time it takes to reach each lower level. Seeing how long it takes to drop that first foot will probably be your most important number!

If the pond used to reach full pool after heavy rains, but NEVER reaches full pool now, then I expect your leak rate may be too high for you to reasonably afford to keep the pond full using your well. Likewise, if it drops a foot in just a few hours after all of the surface water has run into the pond, then your leak rate may still be too big.

How deep is your water well? And more importantly, how deep is it to the static water level? Also, how far is the well from the knock-down transformer at your house. Your pump sizing requirements may depend more on the electrical requirements rather than the hydraulic requirements - if that makes sense to your level of knowledge.

Finally, if you think your well pumping plan may work, I would call a couple of the water well drillers in your county or wider area. They usually also install and service well pumps. I suspect a crusty old guy could give you a ballpark estimate over the phone to match your expected pond filling usage if you can roughly determine your leak rate.

Good luck on your pond repair. Do you have any good clay in your subsoils near the pond? You might be able to drain the pond just a few extra feet and pack two 6" lifts of clay along the upper 5' of your banks. That should be less than your 20-30K full re-line bid. But you do need to have clay very close for that option.