Online metals has a lot of useful information on their site even if you don't buy from them. They have weight calculators, hardness tables, melting points and one of the most useful, descriptions of different metals. I've linked to their product descriptions, but if you scroll to the bottom of the page look where it says learning center, there are all kind of useful links there.
https://www.onlinemetals.com/en/product-guide
I get that you just want to try some different materials but you can at least look to those that are easier to work with / avoid really bad ones. When you see the term "free machining" in relation to a metal that is a good thing for the machine operator unless you are a glutton for punishment. Be aware that for some reason good machining and good welding qualities often do not go together, so if you want to machine something and weld it your options are much more limited. In my limited experience smaller local metal supplies tend to lean towards welding supplies in their metal selection.
In steel 12L14 is nice to work with. 1144 aka stress proof is also nice and I've found it more readily available than 12L14 at least locally. 1018 is fairly cheap and weldable, but it doesn't get a great surface finish. I usually end up cleaning it up with some emery cloth if I need it to look nice. "Hot rolled" is similar (don't know what number it is, the local place just calls it hot rolled).
Aluminum 6061 is common and machines well, there are some that are supposed to be better but this is the only one I've used. It can also be TIG welded.
Brass 360 aka "free machining brass" which is a clue
is nice to work with except for the freaking slivers.
Bronze is very similar to brass but it machines differently. I have no idea what kind it was as it was free, it came out real nice though and machined more like aluminum with long curls instead of a bunch of little stabby splinters.
If you can get your hands on some cast iron, it is worthwhile just for the experience. I would describe it as standing in the path of a hot sand blaster.
I've turned all of these except for the cast iron on my little Sherline mill, so I'd expect the 6" Atlas to be capable if working with any of these. I have not played with any stainless steel.
