Alright, so first of all, I assume that you want Windows Server 2012
R2. I mean, it's good to get the latest version when buying a server.
I personally used to build my own servers and PCs but I recently have seen that buying and building doesn't make that much of a difference (speaking about business now). For example, I bought a HP-PC for a dentist customer of mine and it is possible to buy a 3 year, next business day warranty for that thing.
Anything happens to it, you just call HP, they come fix it and go! Costed only like $150 extra! I was so thankful about this about some weeks ago. Two years down the road, the PC just died. Didn't even start! I called HP, no hassle, no nothing.
They sent a technician, he got new CPU, moderboard, RAM, PSU, everything and fixed the thing! I didn't have to reinstall etc. Actually, I didn't have to be on the premise!
It was excellent!
As for the operating system, I would put a Microsoft Hyper-V (it's a barebone virtualization, free of cost) on that thing FIRST and THEN install Windows Server 2012 R2 inside that. Because with virtualized server, you can take snapshots (checkpoints) which will save your ass many times I assue you
For each user connecting to the operating system, you will need to buy a CAL (Client Access License).
If you want to use MS SQL server, you need to buy a license for that too. Depending on the size of the business, the price can be high or low to the customer. Everything is relative I suppose.
"We" in here, we run Windows Server 2012 R2... and use MS SQL but we use the free version. It has some limitations but it's free to use, even for business so depends on the workload I guess.
Athough, with Internet lines so stable these days (at least in Europe), I would personally just use a virtualized server, for example from the place we rent our servers:
http://www.tilaa.com. Works flawlessly. Of course, it depends on the type of business and how critical it is for them to not be able to connect to server in case the line is down a few minutes or hours in a year.
But regarding hardware, I 100% definitely do not recommend you to build your own. Get a HP or DELL but make sure you get the next business day, on premise warranty, which isn't all that expensive at all. You'll be able to sleep well at night.
You can connect Hyper-V to cheap NAS/SAN or you can also have two or three drives inside the same server, running raid. Although, a backup outside of the server is good on a NAS or another server some place.
Let me know if you need further assistance. I'm unsure if you need tips about CPU / RAM etc. or if you got any questions about Windows Server / Hyper-V, I have been using them a good while now, Hyper-V for years so I will try to help.