WiFi will probably choke on multiple streams simultaneously, I would go with wired rather than wireless if you are talking more than 2 streams (more than 1 if high definition).
Storing files on a NAS box would be convenient.
I don't see how LCDs with USB ports will work - that's for connecting to a flash drive not a network. However, some LCDs are connectable to home networks over ethernet, so that might work (have never tried one).
Depends on the nature of the business and how user friendly the setup has to be. If you don't mind ugly menus and awkward remotes, connecting WD TV Lives using wired ethernet to a router or switch, with a NAS box also attached might work. Depends on how many screens/streams the setup has to be able to handle. At circa $160 retail each, it shouldn't be too expensive. Other media players might also be worth a look.
Your system will only be as good as its weakest link. As well as network throughput, you have to look at the throughput capability of the NAS box as well. Some will handle multiple concurrent users/streams better than others.
More details about types of users (eg technically literate), setting, and numbers and types of streams etc might help us give you a better answer.