"Entry level" can mean different things to different people.
For years the MAC10 or M11/9 was it. They were inexpensive and easy to find. They were bullet hoses with crappy buttstocks and the handful of available aftermarket stuff looked like it was cobbled together by a monkey. Looking at what is available now, that situation has obviously changed.
The MAC-series of guns are probably still the most available, just due to the numbers of transferrable guns out there. But for an "entry level" gun, there are a few other options that are at, or slightly under the MAC or M11/9 price point. I'm thinking about M1 carbine conversions, Reisings, some of the registered tube guns like STENs, Stemple tubes, or a couple others. You occasionally see a Spitfire or Commando, which is a Thompson look-alike (sort of), several of which were registered as or reclassified as machine guns. They don't come up for sale all that often, and are sometimes looked down on but the handful I've seen ran well. They usually sell at a discount compared to many other MGs.
Some of these "entry" level MGs are low in cost for a reason. Mags for the Reising are expensive when you can find them, for example. But as the cost of the guns increase, people will find ways to make them useable. I have a set of 3D printed sleeves that allow the use of Promag Uzi .45 magazines in the Reising, and they actually work very well in my gun.