Hi there, and

!
Spring is a great time, I love that everything starts to bloom and green again after the winter!

I like the FishMate filters, and have a few running my tubs, some with UV, some without. They come in all different sizes at DrsFosterSmith, and I would go for the one that filters just the right amount for your pond size.
You have a 500 gl pond, so I would go for one that says it filters 1000 gl. Most of those gallonage are meant in water gardens, so you should divide it by 2 if you have koi/goldfish in your pond. If it filters 1000 gl of watergardens, that makes it 500 gl for fish, and therefore just right for your pond.
The UV part is nice on the filters, and they hold quite a load of filter media as well. The only thing that bugs me with those filters is, that the UV is usually supposed to be AFTER the filter media, but its just a minor thing. Otherwise the FIshmate filters are great!

As for the pump, it says usually on the filter how much the pump should pump max. For a waterfall, since its gonna be a bit higher, I would suggest something a bit stronger than the filter says, since it has to pump the water up a couple of more feet.
I would think that you can use the same filter for the waterfall. Just put the filter on top of the waterfall, and let it run down through whatever you have planned as waterfall.
Or, if you wonna go with an extra filter, just for some extra filter media, get a Fishmate without the UV for the waterfall. Stuff it full with cheap filterfloss from the craft or pet store, and you have a load of extra filter media for the pond.
For treating my water I usually get Prime in a 2 l container at BigAls or other online stores, its enormously concentrated and lasts you a long time. There are all kinds of good water conditioners out there, though, Amquel Plus being another one....
Good luck digging!!