The SKS is a fine little rifle. I have seen tons of these rifles in use (most probably haven't been cleaned since being imported) and I have never seen one of them yet that malfunctioned. They hold 10 rounds and can be reloaded in a speedy fashion using cheap, readily available stripper clips. There is also a ton of accessories on the market for them. Just slap yourself a synthetic stock on one, put a sling on if it doesn't have one, then buy some strippers and Wolf and you are ready to go. The original stocks tend to be a little short for me and the aftermarket ones make them much more comfortable.
In regards to the 7.62x39mm round, I would venture to say that those who call it "puny" or "not a very good stopper" have never been shot by it. This round probably has less effect than a .308 or 30-06, but it wouldn't surprise me if it outperformed some of the smaller caliber cartridges. I would rather have it than a .223/5.56 in cases where the .223 doesn't fragment. Even with FMJ, if the bullet hits a vital area, it will have affect. Add in SP's and it should be very effective. I once had a friend who shot a rather large bodied 8 point whitetail with a Mini-30 and cheap 7.62x39 Russian SP's. The meat processors who packed his deer (they have dealt with thousands) asked what caliber he shot it with because they said the wound was one of the wildest they ever saw. They had thought he used some sort of very powerful .300 caliber range magnum because it had been so severe. That leads me to believe this round has very good potential with SP's. And the SKS's I have seen will function fine with these SP rounds. And they are very cheap.
This gun should make a good deer getter for you, as well as plinker or even self defense gun if needed. I'm not sure what your definition of SHTF is, but if it's home defense, I would rather have a good 12 gauge. I tend not to plan for many of these wild SHTF scenarios that you hear all the time, because I just don't see them happening. But I guess the SKS would be a good choice too for preparing for the unknown. But just don't spend too much money because of this....A SHTF scenario such as invasion by foreign army or mass chaos nationwide is highly unlikely. But do keep a disaster supplies kit with emergency items for stuff such as floods, tornadoes, hurricanes, earthquakes and Winter storms, depending on where you live, as these events are much more likely to happen.