Ok... $400.00 and some paitence you can get whet you need. First off with that budget, quality will come in a used package. For longevity, and reliablilty I'd go with a major manufacture. Not that the majors dont have klinkers... You are just much less likely to run into one.
Glock
Browning
Smith & Wesson
Colt *
Sig
H&K*
Ruger
And probably one or two others I didnt think about.
* Some firearms from these manufactures in my opinion may not be easily repared in difficult times.
Then there's caliber.
.38
.357
9mm
.45acp
.22lr
The ammo in these calibers is readily available.
Okay... soooo with a $400. budget Just to have something for the coming difficulties... I'd browse gun shops, and pawn shopps for the deal that would fit my budget. As another poster mentioned a .12ga shotgun is a must. I agree. There may be times when a shotgun will be out of place, or you and significant other may be split up. So a second gun is in order. A handgun. Keeping within the budget your going to have to own some patience. As with the above handguns the shotgun will probably have to b used as well. Again I'd stick with a few well known brands.
Remington
Mossberg
Ithica
Stevens/Savage
These guns are the work horses of the breed. None are finicky nor ammo sensitive in my experience.
Keep in mind that used is just that... someone else owned it before. Safe queens are a premium, and will cost more. While a old war horse thats looks as though it's been through the wars can be had rather inexpensively. OK so back to choices.
As I say I'd haunt gun shops, and pawn shops until I find this deal... That police/security trade in Model 10 and that older ugly Rem.870 on the rack... both for $400. You may have to kick in a tad more, but not much.
I know it can be done. I did it last week. a 1990's I believe model 10-10 H/B for $210. and a well worn Remington 870 18" for $200. The model 10 looks like heck. But the sction is smooth, timing is perfect, and it locks up nice n tight. The 870 while the stock is a bit bleached, and has a patch of rust on the reciever, and much of the blue is gone. It works smoothly, and chambers and fires every time.