I would run PEX all the way. Just strap it down. As far as exposure to sun light if it's not in direct sunlight I wouldn't be to concerned. One thing you should do is use pipe covering on the cold line in the attic space. This will help to prevent the cold water from getting hot in the summer. (attic spaces get very hot down here in the summer) As far as freezing, if that was a problem down here the exposed water line feeding the home would also freeze.