Yes, flour and water is an age old method for producing a starter, most here will at some time have created one this way. I've done it with both rye flour and with white flour. The process (at least for me) took some 5-10 days to create a satisfactory result. There are lots of other starters that can be produced using different source substances as you highlight but these produce different things. The yeasts in grains, fruits, vegetables etc is (I believe) different. When grapes overripen or rot for example they produce botrytis which is used to make dessert wines like Sauternes. You can make a "starter" from all sorts of things but the characteristics and tastes will differ because of the different bacteria involved. I don't think there is any right or wrong here, just a set of choices to produce different end products.
I like to experiment with things and discover what I can. At one time I created a "starter" in about 2-3 days using raisins as my source, just raisins left in water which quickly ferement and produce a very lively culture. It's a type of starter but not what I would call a sourdough starter. Being sourced from raisins is was a very sweet starter not a sour one. So it comes down to what flavours you want to achieve I think.
Currently, out of interest and as an experiment, I am creating a new sourdough starter by following the "Professor Calvel" method which should produce a useable starter in 2.5 days. I am on day 2 of this process and will post a blog with my findings with photos of the starter at different stages.
In terms of starter pH this is a lot to do with how often you feed it and what with. In the past I failed to feed my starters enough flour and they went very acidic and developed hooch. These were starters made just from flour and water. It's good to understand how to manage and vary the pH of starters so that you have the flexibility to create mild or very sour loaves according to what you, your friends or your customers want.
I was staging at one bakery that would spend 200+ dollars a week on Strawberries to get their 'Strawberry Bread' going... Unnecessary expenses.. xD