Hi Indygunworks, 30 x 40 is a non issue for a 70GPM well unless your well will runs dry quickly. I filled my pond with 10GPM and it is just over 1 acre 12 to 17ft deep. Here is my opinion, not facts on some of the other questions.

Water Clarity: With fish and a feeder you will have some issues from time to time.

Longevity: Pond, hard to say with liner. I know nothing about them except the ones we had in the refinery I worked in, had lifespans of 10 or so yrs.

Quality/Cost,freezing concerns: ? Don't know, I live in the south.

Clay might be needed around the edge to seal but not sure. I would assume it is not needed in the bottom of the pond say as a cushion layer.

Sealing around pylons: They probably should not penetrate the liner i.e. better to have a floating dock.IMHO

Rip rap: yes, helps for control of erosion. IMHO

Fish types and counts: Others with knowledge of your Geographical area will come along to give you some Ideas.

Good luck and post picture if and when you can, as you go.

Life is more fun with a pole in your hand.