No such thing. An AR upper receiver is the same on all guns of common caliber. 308 will be a little bigger on some models because of the bigger bolt and bolt carry group. The latest thing is to use a standard upper for 308 and down size the bolt group.
80% lowers are legal if you are allowed to build a gun for personal use in your state. For you getting your 1st AR I wouldn't go that route. I'd get a good commercial gun to start with. A Colt is a good gun to start off with if you want something hard use. Or if you've know a guy who can do a 80% lower get them to talk you through getting a lower and upper complete.