Another option if you want a sub gun is to save a bit longer and purchase a MP40 tube gun. Great shooter, very smooth but parts and mags are expensive as a downside. I am currently waiting (transfer) on a British C&R Sten, and have never fired one, but parts are easy to come by and mags are super cheap. A real Sten has some collector value, plus parts and ammo are cheap.
If you want a rifle, then save for a M16, which I can't add any more to the discussion then what folks have already pointed out to you.
Most folks don't stop at one but this is an expensive hobby.