I agree, that looks like a fun and sturdy structure to start with! Only two things I can see that are going to be a pain to deal with.
1. You probably want to get that wood floor up off the damp earth unless it is pressure treated. Here in Cali it would have been eaten by subterranean termites already! Check it over for rot carefully and repair as needed. Probably good if it is PT wood, though.
2. You will probably need to deconstruct the entire upper deck area in order to make a functional and weatherproof roof. On the plus side, that is a lot of good lumber to work with! Use it to make a good roof with big overhangs.
Check out @3KillerBs open air coop for ideas for a climate similar to yours. You don't need a fully enclosed coop in your climate.