There is a lot that goes into deciding what to get for a server for a small business. My business works with customers just like this to build technology solutions and many of the times we are setting up a small office with their very first server, so yes it can be quite a daunting challenge it may seem.
First, I would not recommend going with desktop hardware. Given the size of your network, and that this is a business environment, I think you are asking for trouble. The cost of enterprise-class server hardware is not really that much more comparing apples to apples, but it is BUILT for continual 24/7 usage and years of reliability. A desktop system with mainstream hardware is just not built for that kind of usage, and often times lack support for some of the software or hardware features and upgrades you may end up needing!
For a file server, I'd recommend at least a good dual core processor, preferably a quad-core Xeon just to give you all the performance capabilities you can for future growth. 8 GB of memory would be the minimum I'd recommend but that's also quite a good amount so you shouldn't really need to upgrade that unless you add more roles or services to your server. You will also want to have RAID configurations, and if this is going to have numerous connections and business-critical information, then I'd recommend a hardware RAID controller instead of just cheap onboard SATA RAID with most mainstream desktop motherboards. It is not nearly as reliable nor offer the performance as a desktop RAID controller.
For the best flexibility, I usually go with two RAID 1 arrays, the first array you can do with SSDs, but if this is just going to be a file server it shouldn't be using a whole lot of access to the original OS anyways so is kinda wasted. You don't need high capacity hard drives, so I'd say something like WD Black 500 GB hard drives in RAID 1 would suffice for the cheapest option. Ideally, with a dedicated RAID controller, you'd consider something more like 300 GB 10k or 15k SAS drives for added performance and reliability.
The second RAID array would be just for storing data, and you can use something like two WD RE4 1TB or 2 TB hard drives in RAID 1. I recommend RAID 1 highly because ALL of your data is on EACH hard drive without any kind of striping or parity. This means if there is some kind of catastrophic failure, you can just pull one of your hard drives, install it into any other computers, and you instantly have access to all of your data again and are up and running in a matter of minutes. You can't do this with RAID 5 or RAID 10. This has saved several business I have come in to work at because something went wrong with their backup and their server was down. If you have RAID 5, all of your data relies on that RAID controller due to the configuration of the data across the drives, so you have to have it up and running to get your data. Still, no matter what RAID you do for fault tollerance, RAID is not a backup. You need to have another form of backup that is completely separate from your RAID array.
Now, while you can use a standard desktop OS to share out files to your workgroup, this is pretty limiting and you are reaching the maximum size of your workgroup so you may consider needing to upgrade to a full Windows Server OS soon. Yes, it's more expensive, but it also gives you a lot more capabilities if you need to expand such as domain services and virtualization.
For most small businesses that are getting a new server similar to what you are looking for, I recommend the HP ProLiant ML110 G7 server. These systems are a huge value for what they cost, and can be highly customizable as well. You can start on the low end with a Core i3 dual core processor and the onboard SATA RAID for as little as $500, and add in your hard drives. I would recommend the quad-core Xeon models, and go with at least 8 GB of DDR3 1333 ECC Unbuffered RAM. I'd also suggest adding an HP P410/256 MB SmartArray SAS RAID controller, which will give you more performance, stability, and compatibility with different OS and software needs. There are even configurations of this server with dual redundant power supplies, and this is something I would also consider. Around where I live we don't have the most reliable power, and having that redundant power supply has help keep a couple businesses running after they have had one PSU fail due to power surge or standard wear.