Mike, I have given that a lot of consideration lately. In order of importance
1) Remington 870 Express 12 gage shotgun (two barrels, the 18" cylinder on it for defense, the 28" modified barrel to change for hunting)
2) Glock 19 9mm semiautomatic pistol
3) Kel-Tec Sub 2000 9 mm folding carbine rifle
I lucked into the first two, a generous brother and a good friend who had an extra - he decided that he needed 10 mm guns.
So I have a compromise in some ways with the 9 mm round, but I think a good one. I have a lot of ammo, including variety - 00 buck, #4 buck, and birdshot in several loads.
Some hot pistol loads, and bunches of good old 115 gr JHP. The pistol and the rifle are also for fun, and I can afford to shoot the cheap FMJ rounds.
I have eight Glock magazines, both the pistol and carbine rifle can take them, and the 33 rounders are loaded up with hot rounds.
And this entire armory has set me back less than a thousand dollars (would be well under $2K for new items). My 3000 rounds of 9 and 20 boxes of 12 ga cost more. I figure if the zombies come, I will be able to reload and keep firing, and given my ability to hit birds, I will need a lot of shells to avoid hunger.
