I'd throw it in the attic, use pex so you don't have to worry about freezing. Put it in a pan and attach a pan drain and run the pop-off seperately both barely stubbed out the eve of your house, hardly noticeable.
Just my 2 cents, the pex won't bust if it freezes, not much need for a 90, just sweep the bends and plug into a sharkbite fitting on the heater and you'll be good I think. I just helped a friend do the same thing, he was going to add another 40 to preheat for an existing 40, but the heater was outside in a separate garage attached to the house by a breezeway and his daughter's bathroom took forever to get hot water so I convinced him to add one in the attic just above her shower. works great.
Just my two cents. Assuming you don't want to go the tankless route, in which case your local gas company may give you a very nice deal to install one.