If you're looking for an inexpensive alternative, I'd recommend a pre-owned original sport Apple watch, which can be had for around $170. Apple is also offering refurbished models of their newer Apple watches online, which come with their standard warranty.
All of their watches have the same basic functions and same basic physical design. The Series 1 is essentially the original watch but with the updated dual-core processor, although for most basic tasks it will perform on-par with the original. The Series 1 is only offered in the aluminum case with sport band, since it's their entry-level model.
The Series 2 has the same updated dual-core processor, although it's also water resistant as opposed to only splash proof (neither watch is recommended to be used in the shower because of soaps being able to deteriorate the seals), it has built in GPS to track runs without needing your iPhone with you, as well as the 2x brighter screen.
As for the difference between the aluminum and stainless steel, this is all just personal preference. Stainless Steel looks more premium and feels more premium because of the added weight, although it is also prone to scratches. The stainless steel model also seems to go with a larger selection of bands. The Aluminum model is nice too though, and if you want a lighter watch this is the one to get. The aluminum model also does all of the same things the SS model does, so you may just want to save some money and go with this one.
As for 3rd party bands; there are a lot of nice replica bands on Amazon. They are considerably cheaper than Apple's bands, although I will say that Apple's bands have great built quality. For instance Apple sells their sport bands for $50, and you can find replicas on Amazon for ~$9. They looks very close to the real thing, although Apple uses fluoroelastomer for their sport bands which is very soft and supple, and the inexpensive one I bought from Amazon uses rubber. This wasn't an issue for me, so I decided to save the money and go for the cheaper band.
There are plenty of videos on YouTube that you can find of people reviewing replica Apple watch bands.