Since the water will have sat for months I wouldn't worry about chlorine.
I'll add to the praise of swimming pool nets to remove leaves. Specifically,
leaf rakes. The shallow nets, skimmers, aren't too useful. Also
Venturi vacuums work well, especially if you want to get every leaf.
You do need to match the pump and GPH to the UV filter spec. And the UV can be sized according to the pond volume. I would let the UV drive the pump size. The pump spec will also tell you the GPH at different heads (how high it has to pump water).
Now, for what many would consider heresy...
The pump GPH thing is just something people like to say. My best advice is be really careful about what you hear. You have to judge for yourself, but be skeptical. All ponds are different, not everything applies to every pond the same way. I used a 900 GPH pump in a 7600 gal pond for years, but that was my pond.
Better to understand why you would need a pump. Same reasons as you have with the indoor tanks applies to ponds. How much water flow would you need in your 55 gal tank if you had just one Tetra? Probably no water movement, no filter and that little puppy would be just fine. Well, I don't actually know that to be a fact, but you get the idea.
If you're going to be adding 10, 20, 50 pounds of fish in the spring you do have to be ready for ammonia, cleaning, pumps, etc. If you're adding a few 4" Koi to a 1600 gal pond you don't "need" any pump or bio filter. One way people cycle bio filters, as you probably know from running tanks, is by adding a few small fish. Ammonia levels don't get high enough to hurt the fish. The large mass of water dilutes the small ammonia load enough to give the bacteria time to consume it. Just like with a tank the whole pond is a bio filter. The question should be is that bio filter (the pond) large enough for your fish load.
Obviously, as the fish grow, if you choose to grow out the fry, if you choose to feed a lot, your fish load grows. Someday you might need more O2 or more bio filtering.
To know if you have enough bio filtering you do exactly the same as with a tank, you test. You could have 0, 1, 2 or 20 bio filters and that may not be enough. Testing tells you. No one on the internet can tell you bio filter X will always keep your pond safe. You could guess what kind of fish load you're going to have 5, 10 years down the road and try and build filters to that standard. But you still have to test.
Don't get me wrong, I'm pro pump and a good working bio filter is only good. But I think understanding a little of the whys is more helpful, gives you more choices.
Now, for what many would consider utter, burn me at the stake, heresy...
My own personal opinion of the old mat in the falls deal, or even mat things in general, is they are about as poor a filter as possible and still be called a filter. Trickle type bio filters are much more effective, cheaper, and easier to maintain.
How a pond stays clean without a pump in winter is kind of the same question why ponds stay clean in summer without a pump. Or why ponds with pumps are often not clean. Basically because the pump has little to do with clean or not. If clean means clear...a clear pond without a UV filter will have bacteria that attack algae and kill it to eat it, and/or string algae which I believe produce chemicals that attack single cell algae. If clean means no ammonia...its because the amount of ammonia produced is being handled by just the pond being a bio filter without a pump. Ammonia can out gas too.
