It takes lots of sanding (and not just sanding, but sanding done right, so you don't end up with ruts in the wood), 1-2 coats of stain, and should involve 2+ coats of clear coat.

The coatings used are always advancing and you can actually get lifetime warranties on finishes for LP now!

If the majority of the walls are rough and need to be coated, it is probably worth removing it all.

@John Vietmeyer We have them in our primary residence. (1 GSP and 2 labs) They are durable and resistant to scratches, we also picked the ones with a greywash to help hide any scratches from the dogs.I don't recall if ours is 6.3mm thick, but the original padding that was underneath made the flooring sound hollow, so we replaced it with a thicker pad and it solved the issue. 

Whomever told you that you can only do it once or twice was probably referring to an engineered hardwood product (they have a thin layer of real wood over a thicker layer of particle board).If the floors are real hardwood all the way through then you can sand and refinish them many many times.If you're good about it, you can usually get away with a light screen/buff and re-coat instead of a full deep sanding.

I'm not a big fan but many of my friends are.Smart Trust Deed Investment - George Coates.

Prime all exposed wood surfaces and apply two coats of paint on all previously painted surfaces $ 2,850.004) Install rear exterior door unit with all hardware and locks.$ 570.005) Replace concrete pad at the base of rear stoop steps$ 720.006) Remove the dead tree at the right rear corner of the house, haul limbs away and stump the roots

That makes a ton more sense; PM or HM.This IS a place to get help, but I'm not a kid-glove, or sugar-coating kind of guy.The points raised and information I relayed are all legitimate points, which will be helpful to you.

The apartments look they it's been a while since they've had a fresh coat of paint or new carpets, and although I'm not looking to remodel anything, I'm curious, beyond fixing any glaring issues, how much rehabbing of the units would you recommend?   

Now think of a long coat hanger, it not but a fish tape is a lot like it, only longer.