SE18 is the member with the railroads.
#19 IOW, when it rains, you don't want run-off water from the yard flowing INTO the pond, as would be the case with low-lying ground. Not usually a problem, but one that some folks don't take into consideration when they design & locate their pond -- until they suddenly end up with a flooded pond, lots of mud, or dead fish (from pesticides or other chemicals in the yard).
The API test kit (liquid) is what most on here use. I get the extra kit for KH & GH.
Here in the US, herons seem to be the worst predator problem, but some have raccoons, and maybe cats. Snakes, too, but not sure there's a way to protect against them. Folks who have dogs say they are great heron deterrents! But some have also had dogs get into the pond and damage the liner (sharp claws, trying to get out). And if you have a fake heron, be sure to move it around.
I wish I was good at finding and including links, 'cause there are a lot of good suggestions for ways to "edge" a pond on here (and "search" isn't too helpful). I built a dirt berm about 6" high around my pond, with liner & rocks on top. But my clay soil is getting squishier with time and my edge is flattening. Almost wishing I'd gone above-ground a foot or 2 with a wooden retaining wall so it would be easier to sit pond-side and visit with the fish (I'm assuming you're young enough that it won't be an issue for you). But that just goes to show that we each have our own needs as far as pond building, LOL!
Slate sounds nice! There's a way to construct the edge so that you don't have bare liner showing between the edge and the waterline. I would think that would also make it easier for critters to get in and out. I can't really describe it.... What you do is have a little shelf that goes all around the inside edge -- maybe about 6" or so from the top, and 6" to a foot wide. The "edge" or shelf should have a slight slant, sloping toward the land (not toward the water). When you "rock" your edge, you start by stacking rocks on this shelf, and continue to the top. The slant simply angles the rocks so they are less likely to fall into the water. But the effect is of a natural rocky shoreline.
Hedgehogs! Cool! Never saw one in the wild!
Main, and most important advice is to do JUST what you're doing now -- starting early, asking questions, and having a plan! You're on the right track (no pun intended!), and you'll probably find that you'll change your mind a lot (which gives you ideas for future ponds!). And keep in mind that there are lots of opinions and ideas out there. What works for one person, may not work for another for whatever the reason, so stay objective and keep asking questions.