Ken's a pond professional. I'd heed him.
I've only built a few ponds but usually on more or less level ground.
The best filtration depends on details, what's available, etc. Bog / natural filter sounds great but depends on your climate too. I like wet/dry. If you use potted plants inside your pond (in a section that's separated or not from your main section) you might not need a filter or much of a filter. I'd advice into providing a very good flow / stirring, s strong as you fathomably can afford financially; pumping costs $$$; powerheads and bubblers are cheaper and more efficient at stirring..
EDPM liner or Reinforced PE liner (may or may not be 100% waterproof IME) are good options but have their cons. Can't house spiny and toothy fish. I'd consider in earnest a concrete pond / swimming pool like - a lot more labor but could last forever if built well. With or without a liner membrane. Or a fiberglass inground pond - see them on Craigslist sometimes for cheap.
I'd make the bottom come down to one or a couple low spots in a conical fashion to have all the dirt gravitate towards there and attach drain pipes with valves, large diameter like 4", so once in a while you turn the valve and drain all the dirt.
Same goes for the stream.
See my 25K videos (which IS rubber liner) for the fish selection - koi + a large and diverse Co.