Yes, all chrome wheels are "plated", it is a process that involves several layers of different metals. There are still some chrome steel wheels out there they are cheeper and heavy. I would say 90% of what is out there is aluminum. I think any chrome wheel you would buy for your Mustang is going to be aluminum/chrome. The steel/chrome wheels are mostly seen on trucks, they look almost stamped out of steel very generic looking.
As far as quality, of course you get what you pay for, I suggest buying imported wheels in other words they were chromed somewhere outside the US, they are better quality. And never buy or have wheels dipped (mostly done in Cali) and expect the finish to last. Also know you have to take care of them and protect the finish on any wheel or it will deteriorate.