How pricey does the media need to be for a typical bacterial culture experiment?
It looks like Merck have different levels of quality control for their products. MQ 100 being the lowest up to MQ 600 being the highest. From what I can see the MQ200 product here has defined composition (of salt, tryptone and yeast extract in g/L) whereas the cheaper MQ100 product does not appear to have its composition ensured.
I am sure your cultures will grow in cheaper media but perhaps you would theoretically lose out on some replicability and perhaps if you were doing metabolically focussed experiments you would want to be certain of your media composition.
As for water, I have always used distilled water which is then autoclaved after you add the media with no problems.