Before I go collect rescue chickens the absolute necessities are:
Somewhere safe, dry and draft free they can call home
Secure, roofed and wind proofed, with a dry floor. Bedding is needed here, hay or straw do fine, but pine needles are probably better pest wise. I do not heat my coops - these girls come from a factory with round the clock heating. They do perfectly fine once they can get out of the wind/rain. In fact, they often still choose to be outside on wet, windy days, so once their coop is dry and draft free they have always done fine. I do fill a corner with straw as I feel it is warmer for them to snuggle into their first few days, which they do. I don't need to provide roosts for a few weeks as they do not be able to get to them until their strength builds. I need something to hold feed and water in, and a few nest boxes (depending on how many chickens I am taking, one per four seems fine). Cardboard boxes will suffice until something more solid and permanent is arranged. Hardware cloth is your friend. Cover all exteriors with hardware cloth if at all possible. They will need a run also of course, but initially you could put up a temporary run which would need to be supervised until something more permanent.
Feed, Oyster shell & Grit
My local layer feed is 16% protein and I want to give them more than that so I provide an 18% grower feed which I steep in water the night before. For the first few days I also give them greek yoghurt and tinned cat food. They love this and it can get their appetites going as they can be very fretful being in such a new world. It takes time before these girls will see tuna as a food source and not something going to eat them. They initially run away but after a few weeks they love tuna too. After the first week they get only chicken feed available always, and a high protein treat once a week. (I am strict with this. They have been through enough, I do not want to add to a weakened system by diluting their nutrients).
A Sick bay
This simply needs to be somewhere you can put a chicken if it is ill and the others are pecking at it. Blocking off a corner of the coop is perfectly ok. The move can bring out all kinds of things in chickens and the first few weeks are always a time of very close observation.
Poultry Vitamins
They need to build up their strength in the first few months. Once a week I add poultry vitamins to their drinking water.
Sudocreme
The rescue chickens I collect will often be partially naked. This causes pecking (and is mostly from pecking). Sudocreme seems to help - a lot.
Vaseline
Scaley leg mites always present to some extent. To Vaseline their legs now really helps before the problem gets worse.
Permethrin Powder & Spray
Lice and mites often a serious issue upon arrival. Dust each chicken as I am letting them into their new home. The spray is used 10 days later when the chickens get dusted again I spray the coop.
Dog nail clippers/nail foil
Overgrown nails (and beaks/crossbeaks) always an issue. I do this day one also - maintenance required.
Corid.
With my rescue chickens it will be their first time on ground, with yours, it will be new ground. So Coccidiosis is a possibility. If this strikes, you don't want to be going looking for corid, you'll need it straight away and time is of the essence.
Epsom Salts
The stress involved in the move can and does bring on egg binding and other reproductive disorders. An epsom salt bath (or many) is often required within the first few days.
The above are the things I use immediately on collection of the rescue chickens as I am unloading them. They get dusted with Permethrin, nails get clipped (they have been living on wire so terribly long nails), vaseline on their legs for the scaley leg mites, sudocreme on any bare, red or bloody patches, have wet feed ready and clean water with added poultry vitamins alongside oyster shell and grit (beside food so they can find it straight away). The Permethrin spray is for the housing, I don't use the spray on the chickens.
I hold off and wait to see if I feel I need to worm. I know depending on location and weather conditions this seems to be required in some places so a good broad spectrum wormer is probably necessary also.
The above is essential to me before collection. The following have proven to be essential at later times.
Electorlytes - lethargic chicken
Antibiotic Ointment - wounded-open to infection
Tylan 50 - respiratory illnesses
Amoxicillin - internal infections/reproductive disorders
Calcium Supplements - laying issues
B Complex vitamins - deficiency
Bandages
Gauze
Vet wrap
Scalpel (bumblefoot)
Sulfa powder
Manuka honey
For crop issues (which are many with chickens that have only ever had access to pellets and are now on the ground): this article has saved me, and some of my chickens!
https://www.backyardchickens.com/ar...d-sour-crops-prevention-and-treatments.67194/
Also, on hand, I strongly recommend this forum. It should be on every chicken owners list of necessities. Even if you never need to post, you will so often find useful info either in the articles or within the forums.
As a final note, I tend to the 'essential' things I listed above straight away. I find it best to nip things in the bud, also, If you are needing to constantly upset the new chickens by trying to tend to these things over time in the first weeks they will be very skittish and afraid of you. Their priority when they see you will be to hide or get away. My advise is: do as much as you can, as soon as you can, then show the new girls you are someone who feeds them, not someone who upsets them. They will become really friendly really quickly

I am sure I have forgotten to mention some things, but I will think more about it and add to this post when anything comes to me. What an exciting time for you
My avator is my little rescue Dolly (she is a real little dolly bird). I took that photo in her first week with me. I will need to take a new photo and upload it. She is a fine bird and I am only her proud human around 5 months now!