Jesse:
If you do a liner, you can do it 100% of the way to the top and pump water out to water the trees. I'd recommend doing that vs. stopping short of the top. That way you have control over the water.
I agree with you; do it yourself than pay someone that has no experience in doing it. I feel you will be money ahead and you will do a better job because you are invested in doing a good job for your dad, it's not "just a job".
Look into the liner material from here.
https://www.btlliners.com/aquaarmor It's a LOT lighter in weight than a typical 60 mil pond liner and is a lot tougher too. Being made in Oregon, it might save you some $$ in shipping costs too. They have installers scattered around the USA and the world, give them a call to see if there is anyone local or at the very least ask them if they can offer help over the phone or on-line.
A customer of mine installed one about 7 years ago in rocky ground in Arkansas. He has a pig population on the property too, and he installed it as a 20x20 or 20x30 pond for wildlife. All he did was bury the edge of the fabric and left the rest of the liner unburied. It is still leak free about 7 years later without any extra care on his part. He tried to remove all the rocks but said he couldn't get them all.
