Welcome!
Chickens really are pretty basic animals - but you can make keeping them as involved/complicated as you wish since there are plenty of "extras" one can add to the whole thing, lol. I have found that those little "extras" reveal themselves pretty well once you put the birds in the coop and start encountering little things you didn't think of before that might make life a little better/easier for you or the birds. Your list will depend a lot on how you intend to keep your birds....and everyone has their own list of things that they want/need in a setup.
"must haves" for a coop would be:
* ventilation, ventilation, ventilation!
* draft free
* security - chickens are most vulnerable at night when they are roosted, so keeping predators out of the coop is key to bird safety - predators can get through even the smallest of space/hole/what-have-you
* roosting space
*nesting/laying "boxes" -- while some may argue this isn't a "must" because in their absence the birds will find somewhere to lay (ie the floor) I put them under must because who wants eggs scattered everywhere?
* a way for chickens and humans to get in/out of the coop - with a shed conversion their is likely already a "man" door, so you may only need to add the pop/chicken door
Things that are nice to have are:
"poop boards" under the roosting space - saves SO much work in cleaning and keeps the coop as a whole that much cleaner
lighting of some sort - even one of those batter operated light setups if the coop is out of reach of electrical wiring - winter is a time when it is dark when I leave the house and dark before I get back, and our barn (which recently also became the chicken coop) is fitted with a couple of these which makes doing animal chores before/after work MUCH easier
Things that don't have to go "in" the coop, but must be provided somewhere (ie either in the coop or run or just plain outside if you aren't using a run)
feeding/watering stations
Will you have a run or will you free range your birds when they are not confined in the coop (assuming they will ever be confined to the coop - some leave that open 24/7)?