Are you going to pressure wash the roof decking, too? I hope not!

Are you planning on removing the garage doors? To make this space really useful, you will need to insulate it. Plus, you will need to first treat this wood for insects and insect eggs or you will be constantly fighting (and maybe even if you treat it) spiders, etc. If I were you, I would insulate it and sheet rock it in and then add some wood for the "look" you want without all the problems. In addition, just looking at this "framing" you probably will need some additional supports to ensure you have a safe situation. Might be more than you think so get some professional advice on site to ensure you understand all the pitfalls. Better know now, than find out when you are knee deep in unanticipated problems.

You need to use an outdoor product if you are not insulating, hope you live in a very warm climate , doing laundry in a cold garage is not fun not to mention freezing water lines.

I would still have a treatment done on the walls before sheetrock, just to make sure the little critters are done for! Also, check the structural so you don't have a weight problem with the sheetrock. Good luck!