Hi GW,
A tricky situation. Firstly, I guess, at the end of the day it depends on how invasive the remaining trees are and how close their roots are to your intended pond.
Not sure on your liners over there, but the stuff over here wouldn't take much to punch a hole in.
IF, I was going to attempt that, I would probably go concrete with reo (reinforced mesh).
But hard to say without being there.
The other consideration of course is how much debri will be dropped by the trees (leaf littler) and the like as you'd want to minise that or at least keep it to a level that's easilily managed with a net.
Finally, if you intend to grow water lillies, then you don't want continuous shade - I believe 4-6 hours direct sunlight min is recommended. I put shade cloth over our pond as my wife wanted ferns and the like around it and now our lillies look a lot sadder and won't flower
