There's only 4 hens left, all around 4 years old. I think I can pull something together in the next few days, I already have a large (albeit empty) shed and a heated waterer that I used to use. I can easily run to the local tractor supply/feed store and pick up all the supplies I need.
		
		
	 
	
		
	
	
		
		
			What do you guys think? I'm torn here. I really want to help, but I'm gonna need a lot of advice from y'all about treating these chooks for whatever health ailments they're bound to have...I would bet my life that they definitely have lice, mites, and roundworm at the least. If the plan is to go forward with this, what would be the best course of action? Best things to pick up?
		
		
	 
	
		
	
	
		
		
			@chriswodog and 
@Kusanar - thank you both for the useful information!
Any ideas for heating? The 6 x 7’ shed that I have now is not insulated, and temps get down to below freezing here at night. I do have a heat lamp but suspect that won’t be enough. Putting down pine shavings should definitely help but I worry it won’t be enough to keep them warm. Any suggestions?
Also, I have an old container of DE that I’ve used in the past for dusting chicks with lice and mites. How effective might this be?
		
 
Photos of your shed?
I don't think you will have to provide additional heat.  The shed will provide shelter and protection from wind/elements.
If the shed is ready, add a deep layer of bedding - pine shavings, straw - your choice.  You've done this before...you got this!
You have a heated waterer, all you need is a feeder.
For roosts, depends on the health of the 4yr old ladies, if they can roost a simple 2X4 secured across a corner or the back wall will be sufficient.
Nesting boxes?  You can buy those premade - look at 
TSC or online.  A deep dish pan or two in the corner will work too.
You can use a Permethrin based poultry dust or spray to treat for lice/mites.  Since it's winter, dust is better, imho. As for worms...you can use Safeguard liguid goat wormer or Valbazen.  We can help you with the dosages.
For food.  They are 4yr old hens - you can feed layer feed or an all flock feed.  All you want is a nutritionally balanced poultry feed.  If they are in poor condition, poultry vitamins in the water for a few days or you can add them to a wet mash.  It takes time for chickens to lose condition and it will take time to build them back up.  It's tempting to want to get treats/offer goodies, but they need to eat their normal feed for the most part.  Provide oyster shell and poultry grit free choice.
As for not having a run, that may be a little more difficult - what is your yard situation - do you a fenced in yard? 
Once you get them settled, inspect each one.  As you see any issues/concerns, post them, ask questions.  We will try to help you in any way we can.