I second the Mosin/Mauser idea. Either one is a great weapon and can do the job.
Here's what I did with my Mosin - shortened the barrel, refinished the metal, lightened and smoothed out the trigger (added a return spring as well), added the long eye relief scope, added a bent bolt (made from the bayonet), altered and refinished the stock, and added a rubber buttpad. My aim was to make a scout rifle.
All in all, I have about $200 in the weapon. As previously stated, ammo is cheap and plentiful (at least for now). As far as ammo availability, in a shtf scenario, what you have on hand is all you can count on, so whatever caliber you decide on, buy as much as you can.
The 7.62x54R is comparable to the 30-06 as far as ballistics. I take my rounds, pull the bullets, reload them with a consistant powder charge (the milsurp stuff varies by as much as 5 grains) and re-use the bullets, and with that I can get 1" groups at 50 yds from a bench. YMMV, of course.
Good luck with the search!