"My vineyard friend has suggested we purchase a used excavator and a dozer and do it ourselves."

Rental rates on earthmoving equipment were quite cheap during the Covid bust when I priced them in my area.

Unfortunately (for my pond building aspirations) my business has come back and I am working too much to take time to excavate my ponds. Therefore I don't know how much the rental prices have escalated since the bottom.

If you bought it and it breaks, you pay to fix it. If you rent it and it breaks, THEY pay to fix it. (Assuming you weren't abusing the equipment.)

Good luck on your ongoing pond saga!