the .243 win is a good choice, as well as the others. not a stupid question.
as far as "where are you gonna get ammo", i guess we should all go back to the '06 and .223rem (that was sarcasm).
If I was you, i would lay out what i want to do with the rifle. decide if its going to be a practice rifle, a walking varmit rifle, a long range hot-rod, a predator rifle or a big game rifle. then think about how many rounds that your gonna send through it a year, and when you can afford a new barrel.
If your going the practice route, you might want to look into the 6BR.
walking varmit/practice- .243 win, 6XC
hot rod long range yote gun- 6mmx.284win (of the fast 6's i think this is the best)
big game-i dont know, 6mm are coyote guns