Could I use beer instead of/as well as water in my sourdough?
You can easily replace the liquid in most bread recipes with beer. This can have a very pronounced effect on your final dough as there is a lot more chemical and biological fun happening in beer than there is in water. In my experience, the dough with beer will usually rise faster than a similar dough with water. Generally, the flavor difference won't be that pronounced (usually a much more "bready" flavor, unless you use a beer that otherwise has a strong flavor, such as an IPA). 

The acidity of the beer won't actually have that much of an effect on your final dough as the ph will be a weighted average of all of your ingredients.