If you are thinking of an ACC556, make sure you get one that made by Ruger, and not a Mini-14 that was later registered and converted by someone else; Ruger will only service factory ACC556s, not conversions (Ruger will be listed as the "manufacturer" on the form 3/4).
Personally, however, I'm not keen on ACC556s as a first NFA gun. Not that they aren't good guns, but they are not as user friendly as other MGs. Parts are not as available as many other MGs, mags are on the pricey side (and can be a little hard to find in the 30rd flavor), and if you're heavy on the trigger, changing barrels when you start burning through them may become a problem. Again, I'm not saying they're bad guns (they're not), but understand they're just not as user friendly and easily serviceable as other MGs in roughly the same price range (for example an AR15/RLL conversion or an Uzi).