As long as you are of legal age (in your state), have no felony conviction, and can pass the NCIS check...and depending on your state/local jurisdiction, you should NOT have any problem buying a lower.
It DOES NOT MATTER if its "technically isn't defined as a rifle or shotgun and can be made into an AR pistol". The lower is sold as a long gun, with the intent that it will be built into a rifle. You CANNOT legally build it into a pistol or SBR. To do so, you must first complete the NFA application to register the lower as a pistol/SBR, and once approved, you can then build it into a SBR/pistol. (But good luck getting the NFA application thru ATF approval...you may need your local CLEO approval to do that, unless you know how to create a trust.)
BTW, if you intend to own a firearm, you will have to know your laws, and local ones. Accidentally violating or ignoring the laws here will get you in serious trouble. From the experience of many here, including myself, firearms laws are the only regs where the govt doesn't forgive ignorance of the law.
Another way to bypass all of this is to join the US Military. They'll give you a kickass fully automatic weapon to train with, just like my own experience some 18 yrs ago. You don't own it and can't take it with you, but you get to play with it and lots of other cool stuff.
(Edited to correct "stripped lower" references.)