I love the recycled ideas. Corner shelf is a good use for nestboxes. We almost bought a tall narrow bookshelf to lay down on its side to use as nestboxes but it wasn't tall enough for head room IMO once it set on its side so we didn't get it. I have an Ameraucana that isn't heavy but she stands very tall and statuesque. The book case on its side would've made 6 nestboxes but I knew the Ameraucana wouldn't have good headroom. My current nestboxes are almost 3-feet tall - it came that way prebuilt at the feed store. I like giving my chickens more room rather than just getting by with minimums.
Your costliest expense with be the hardwire for the run. Are you planning some free-range? We have 64 sq ft of run for 4 hens but they free-range from dawn to dusk and just use the coop for laying eggs or going to roost - oh, and they hide out in it during the heavy rain downpours but very quickly exit to splash in the mud puddles when the rain lightens up LOL.