things I would consider getting a 'shed':
1. Perching space - I suppose 10" per bird, although some of the breeds you are considering are slender. Do you need to allow for the fact that '10 birds' will probably become 15-20 because you like them so much. Also give about 10" to the shed wall.
2. Convenience. Can you fit the perches at eye level. The chickens like it up there, and it is useful/convenient to fit some dropping trays just below. Easy to clean! Remember to allow room for a ladder of somekind - although even our heavy birds flew up (not wing-clipped) when we forgot to put the ladder back
![happy :)](https://chat.allotment-garden.org/Smileys/green/smile.gif)
. The chickens will perch lower if that is what you do - you may just get more back ache cleaning!
3. Space/shelter - are you going to keep them shut in for long periods during the day or do you need the shed to provide shelter and keep the food dry during bad weather (the bigger the better), or is there other convenient shelter/feeding stations in the range? How much of the shed inside is going to be taken up with food bins etc.
4. Nest boxes - internal will take up room (say 14" x 4ft) if you can't make some external ones.
5. Draughts - experience so far tells me that it is humans that are worried about these, not chickens. So I see nothing wrong with nice cheap featheredge (about 1/2 price of shiplap for complete sheds, even cheaper DIY). Featheredge will be draftier, shiplap very wind resistant.
6. Apex etc - whatever you like!!!
In the end, the sheds I looked at seemed quite expensive since I was going to modify them so much, so I opted for some DIY with pressure treated softwood...