Much depends on what you see yourself using the lathe for in the future I suppose - if you're thinking to make brass/aluminium parts, then by all means get some practice with them. But if (like most I suspect) you're going to end up working mostly with steel there is no real reason not to start with it. I can fully understand why you might want to start with 'forgiving' metals because that was how I started when I got my first lathe maybe 15 years ago. It's a good strategy for learning basic control of the machine perhaps, but that's the easy bit - more difficult is getting a feel for turning speeds, feeds and tool geometry. These vary widely between different metals. A tool for turning aluminium is quite different from one for turning brass, and neither is optimal for steel. Speeds and feeds are different too, so experience with aluminium or brass isn't directly transferable to steel. Well, that's what I found anyway with HSS tools. It's a bit different with carbide if that's the way you're going. Your ML7 is a capable machine - just take light cuts (say 0.2mm in steel) to start with if you're nervous, and you'll gain confidence. The machine will tell you if it's unhappy long before you break it.
It's not a great idea to start out with salvaged metal because not all alloys, even if nominally the 'same stuff' (eg brass, aluminium alloy, steel) are equal. The first bit of steel I tried was a bar rescued from a skip - I now realise that it was an extension bar for an SDS drill - and it turned rough as a badger's ar$e. I though it was my technique, or the machine. Steel's steel eh? Not so.
If you do want to practice with brass, CZ121 is the 'gold standard' for machinability. For aluminium 6061T6 works well. For steel EN1A is about as good as it gets in my experience.
Suppliers I have used not mentioned by others are
The Multi Metal Shop and
Live Steam Models. The former is currently offering free delivery which is great for small quantities. I don't know about delivery costs for the latter because they're local to me so I pick up, but they can be cheap because they seem to price on the basis of what they paid for it. So if you want an odd size that's been sitting on the shelf for 10 years...
Good luck, Bob.