I don't know much about your specific software to know exactly how to make a good recommendation really, and others may be able to chime in if they have more information that is pertinent.
From what I can tell you are mainly using the server as a database/file server. If you aren't very familiar with server hardware (and don't want the responsibility of maintaining/troubleshooting/supporting said server hardware) then I'd definitely look into a pre-configured unit from HP. It really is a matter of preference on which brand you choose, but personally I've used HP and really like their stuff. They have great servers for the cost, as well. Many will say the same thing about Dell if they have used Dell previously, so it all comes down to personal preference.
The size of your network doesn't seem like it will require a whole bunch of processing power really. A single quad-core Intel Xeon processor with hyper-threading would suffice, but I'd recommend always keeping in mind future expandability and upgradability on your server. It is always more costly to get only enough for your needs today and then have to rebuild your entire server in a year as it's not meeting your new demands and your original configuration doesn't support the additional hardware you are needing.
RAM is always good to get a little more than you might seem to need at first. It gives you the headroom to grow easily and the most cost effectively as well. Keep in mind that you will need to have more processing power and RAM available if you are planning to utilize virtualization to get the most of of your server.
With a file/database server one of the big bottlenecks is often the storage subsystem meaning your hard drives. I've seen it before where people spend thousands on way more processing power or additional RAM than they can ever use, but go with low quality standard hard drives and they don't perform near the way an enterprise solution should and their entire system is crawling along because of it.
The speed and capacity of your hard drive storage really just comes down to your budget. At minimum, I would recommend getting two sets of hard drives, each in their own RAID 1 array. The first set of hard drives would hold the operating system and programs for your server. The second would hold only the shared data or database files to help improve system efficiency and expandability. Ideally, you would select 10k or 15k SAS hard drives for their improved performance and enterprise longevity, but it is more expensive than standard 7,200 RPM SATA hard drives. I don't know how much actual data you have to store, so that will also make a big difference in cost. Having 10 GB of shared data would make more sense to put onto two very high speed 300 GB SAS drives or even SSDs but it would be quite costly to try and put 3 TB of shared data onto high speed SAS drives.
It is important to get a dedicated hardware RAID controller and not just a basic integrated SATA controller. If you are storing business critical information then RAID will be necessary to allow your server continual up time in the event of a single hard drive failure, and for the best flexibility, performance, and reliability the additional money spent on a hardware RAID controller with onboard cache is well worth it. I'd also recommend looking into redundant power supplies as it can help to keep you from suffering business down time in the event one power supply fails.