Custom assembled NAS for ~40-50 users

40-50 users at the same time or different times? That will change things a lot. and how much storage will you need?

For just data storage, you don't need anything fancy, you can even use regular desktop drives in RAID 5.

You'd be able to use the computer for a website as long as it's not a heavy use one. Even then it should work OK with enough RAM. But I'd run the web site files on a separate drive to keep disk operations lower to keep down seek times when data is fetched.

RAID 1 will be slower than RAID 5, although RAID 5 needs at least 3 hard drives.

RAM, at least 16 gig for that many users for a server.

If this is not a mission critical system, you can use any computer. If mission critical I suggest a true server with quick support and redundant power supplies. If you are on a low budget you can use a regular high-end system with a good UPS. You'd want a reliable power supply and motherboard, although you don't need a high-end gaming setup. Something in the 400-500 watt range and a motherboard in the $100 range.

RAID 5 is not always greater performance than RAID 1 and actually in many instances can be slower performance for varying sized workloads. Still, for a NAS with a few hard drives it is going to offer the greatest capacity with the fewest drives. However, when you start getting to six hard drives or more, I'd always recommend RAID 6 over RAID 5 due to the increased reliability.

Anyways, my first question here would be, why are you considering a basic custom-built box for a NAS that is going to be critical storage for fifty users? This sounds like something that is pretty business-critical and should instead be considering something with support options or at least enterprise quality. You can still go with a NAS for basic file storage if that is all you need the box for. For example, a Synology desktop or rackmount NAS with support for four or five hard drives can be filled with four 2 TB hard drives in RAID 10 giving you 4 TB of usable space but also offering fault tollerance of up to two hard drive failures plus twice the throughput capabilities of a single hard drive. You could also go with a standard server system running your storage if you need it to do other things such as domain controller, website host, FTP, application host, etc.

I have seen several businesses who have not thought about the whole picture of their storage needs and utilization when they go and get a NAS and it ends up running terribly. This is because of many reasons, but there are several things here that I would suggest looking at to ensure whatever you get is actually going to work properly for your needs.

First, you will be limited by the network capabilities probably before you are limited by the throughput of your hard drive storage. A single gigabit connection to your NAS for up to 50 people to share having to read and write data is going to make things come to a crawl when you are doing graphics design work. I would recommend looking into something with multiple ethernet adapters and then using link-aggregation to give greater network throughput. Unless you are using Server 2012 as the operating system of your NAS, though, this will require managed network switches which support LACP for NIC teaming. They also have options for 10 GbE capable NAS upgrades which also would require 10 GbE compatible switch to connect it to the rest of your network.

Next is your hard drive throughput. Again, if you are putting all of the data on just a single hard drive (or two drives in RAID 1) then you are running up to 50 simultaneous users reading and writing from the same single drive which will possibly make things crawl. Leverage high-performance hard drives such as 15k SAS drives or high-capacity (and more importantly high-endurance and not cheap desktop) SSDs which offer greater speed capabilities than a single desktop 7k RPM SATA hard drive. You can also increase the number of drives in your RAID array to gain greater throughput. Four hard drives in RAID 10 have nearly twice the throughput as two drives in RAID 1 and also offers you twice the capacity. Of course, this is also expensive as you are adding more hard drives.

Processing power and memory is another concern that depends greatly on your intended usage. Again, if this system is ONLY going to be for storage, that doesn't need a lot of processing capabilities. Even enterprise NAS pre-built units by Synology for the environment that you are talking about run on a simple dual-core processor and average 2 GB of RAM. However, I'd still tend to look at these as the "minimum requirement" for their environment. If you are going to do more with it than just storage, such as website hosting, domain services, running network applications, print server, etc. then you're going to need to run a more robust operating system and hardware capabilities to do all of the other tasks. I don't know what you have, what other services you may need, so I don't know for sure what to recommend on this part.

And lastly, your storage controller is something else to consider. Again, pre-built systems such as the Synology NAS unites utilize software RAID through their specialized operating systems. You can also do software RAID through other operating systems such as FreeNAS or even within Windows Server using Storage Spaces. However, software RAID may not offer quite the performance capabilities of a hardware RAID controller especially if you are doing more demanding workloads like virtualization. Software RAID can also be a little more complex to recover or rebuild in the event of a drive failure in the array, but that will vary greatly depending upon your array, drives, failure type, controller type, and software.