In a business environment, I would not recommend using the network shared storage feature of the router. This method has little access control, reliability, or performance for that many simultaneous connections. Going with a server (or at minimum a NAS) is going to be the right path.
Setting up a whole new business, all the network, and the computer systems, in a proper reliable and secure way can be quite a task and honestly before any recommendations that I might make I’d have to say, CONTACT A PROFESSIONAL. Even if it is someone you know or a local business you trust to come in and help with recommendations, consulting, and assisting on some of the parts that you aren’t familiar with. It is going to be worth it in the long run to get an expert input than having to have someone come in later and replace brand new stuff with other brand new stuff, and redo a bunch of configurations, because it wasn’t set up properly in the first place. I’ve been there with several small businesses in my community which unfortunately have done this same thing, and it costs more in the long run to fix.
With that said, there are some things that we would need to know more about the business, the type of work and systems that they are needing, the type of network features that they are needing, as well as how it will logically be designed. So, lets start with some questions:
1) What sort of business will this be? The system, storage, and network needs for a business doing general sales (needing to access only small documents or pictures, and working basically within web browser or lightweight Office software suites) is quite a bit different than a business doing something like video or design production work.
2) How do you envision the overall network design? For example, will you have access for the public to get online somehow? Public computers, or wireless network for them to connect to? Will you have multiple departments or areas that need to be segmented apart?
3) What types of files, data, and other information will you be needing to store and for employees and staff to be able to access?
4) Do you have access to be able to run ethernet cabling as needed?
5) Roughly what is a budget that you have for setting all of this up?