Free! Love it already!
As to modifications, I have read many comments on the bad design of the pull out tray. My suggestion is to put in a wood floor and use a dry deep litter bedding. I am a big fan of deep litter as it has worked really well for me so far. I put linoleum on my coop floor underneath my dry deep litter bedding, but I'm not convinced that I needed to go that extra step. I picked up a 8X12 sheet of linoleum on sale for about $25, so I decided it was worth the extra money. In theory, it saves your floor from rotting out. In practice, my dry deep litter is in fact dry, so I don't think there will be much rot in any case.
As to nest boxes, I just mounted some Dollar Tree $1 plastic dish bins on some rails for my hens. It works great for them. If you need to clean out a nest box, you simply take the dish bin off the rail and clean the bin. The nest boxes should be lower than the roost for the night. If not, some chickens may prefer to sleep in the nest box (which is higher) than their roost. Then you have a problem with chicken poo in the nest boxes.
I think you are really lucky in that free coop find and with a bit more work, you will really have something nice for a small number of birds. Hope you post some pictures of the mods you decide on adding and how well it works out for you. Best wishes.