They all come with mounting brackets. I doubt you will get the higher flow rate if you're restricting the input to 1/2". Also, according to this http://www.marchpump.com/beerpump/
you'll get the highest head from the 815 (larger impeller). If you can swap out the 809C impeller with this one like you can with the regular 809, I definitely would. It helps a lot with cavitation problems like others have mentioned.
I don't know how you'd get stagnant water in either pump if you mount them with the input on the bottom. You can also mount both horizontally as long as the bracket is on the bottom, which is more convenient for me, and I haven't had any problems with leftover water after cleaning.