Welcome to GoY Beesue.
when we did our pond [which is raised] we bought the best butyl line grade we could afford. The stronger the liner the longer it should last. Ours went in in Feb 1996 and so far is still working well. We used old wool blankets for our interliner as we had a large number to hand. The modern ones are often similar in composition to carpet underlay.

I would also put a layer of sand 1" minimum on the soil too after you have removed as many stones as you can. The way we checked for stones at the base of ours was to wear thick socks with plastic bags over to keep them clean. We then walked slowly over the soil and found loads of stones we couldn't see.

I assume you know to have a shallow end of the pond for things to walk out or lots of stones/pebbles making a beach.