I'm just reiterating what a lot of the smart people on here have said, but don't skimp out. I love tinkering too and believe in knowing products I sell inside out, but would never build my own server. If you're comparing servers based on specs like CPU support and amount of RAM they come with you're missing the point.
As an example, proper servers have out-of-band management. That's another network card and, essentially another little PC inside the server. It has a web interface and can control the server. This lets you do awesome stuff like reboot a server, watch it POST, enter the BIOS and change some settings and save. It also lets you go in and boot a server that's turned off. This can all be done over the Internet.
If the client decides to virtualise you can just download the custom version of ESXi (or whatever product you're using) that has the management agents built in. This allows you to get notified if there's a hardware problem.
The drivers are verified for server OS'es, and they're thoroughly tested.
There are lots of other similar reasons. You don't want to be the only person in the world seeing an incompatibility between a certain brand motherboard and a RAID controller (because no-one else in the world has ever tested those two products together). Good luck to you trying to convince the motherboard manufacturer to investigate that bug and put out a fix in quick time, just for you. And trying to convince the RAID controller manufacturer the same thing, since it could be their fault too.
In the server world it's just not worth it. Reliability is king. Get yourself a good brand, off-the-shelf server that's had all the components tested together. Any fun you have custom building something, or any money you save just ain't worth it when you start to have issues and the client has continuing downtime. That's a great way to lose clients.
As for brands, I only ever sell HP. I know their line-ups and software really well now so supporting them is easy (again: keeping things plain vanilla). Reliability in HP is awesome, and support is very good if you ever have a problem (parts are pre-shipped overnight, or brought out to you by a local repair tech). The only thing I don't like about HP is the premium you pay for their hard drives.
It sounds like you only need a fairly basic file server. If $$$ is an issue consider quoting an HP MicroServer. These are a bit bigger than a NAS, but have a lot of the goodies you get in a proper server like a solid RAID card, ECC RAM and even out-of-band management (HP call theirs ILo).
Dell are also very good, but I can't comment further due to very limited experience with them.
Yes - definitely avoid Fujitsu, and I'd add Acer to that list (I expect others will disagree with this but that's been my, admittedly fairly limited, experience).