Hex receivers are nicer finished than the wartime round receivers, which can have what looks like file marks all over them. But is possible that the hex receiver rifle is totally mismatched, as nearly all the MN rifles around now have been totally rebuilt at the arsenal after WW2 and you might have parts from all kinds of rifles reassembled and refinished into a serviceable but not original gun. The rifle you get with the hex receiver might be identical in every way to the 1943 rough as a cob receiver except for the receiver! For me then, the bore condition is paramount if you wish to shoot the thing. Frankly, with the MN rifles now, I think it is better to go to a show or gunshop with an inventory, and hand pick the one you want, yourself, rather than rolling the dice with a mail order. They are all a little different and you really don't know what you will get thru a "hand pick" from a wholesaler. Notable variations from gun to gun, as far as stock finish, stock condition, maybe quality of the metal(but most arsenal reworks have nice blueing) bore condition, counter bored or not, how smooth the action works, how tight, etc. I have gone to a table full of MN rifles and picked thru and handled a dozen before making a pick. As for ammo, ball for shooting, more expensive soft points if you plan to hunt with it or something. Some people do, so a couple boxes of SP can't hurt.
Your gun is like your nose, it is just wrong for someone else to pick it for you!