Anything with moving parts is going to fail; if you build a low-performing system that relies more heavily on passive cooling, you'll boost reliability there alone. SSD's don't have moving parts, so if you don't mind the cost and lower storage, that will save you on heat and wear as well.
Big name companies like Dell are there selling servers because they have a name behind them; you have an issue, you get replacement parts on short turnaround. You need new drives, you need tech support, you need whatever, you get it. You also pay for it.
You can do something similar with off-the-shelf parts if you stock up on spare parts; it puts the onus of support on you rather than a company with warranty support.
If you have redundancy planned for with backup servers and such you can do fine without spending a lot on server hardware. On the other hand, you can get older hardware from Dell's online outlet at steep discounts, and refurbs or unwanted server systems that were custom built but refused by a customer (or misconfigured) can go for a song on their deal area of the website.
If you're looking for a home server or looking to save money, you could just look at various wikis and HCL's for known-good hardware for the OS you're looking at getting and putting together some beige box solution with some spare parts available if you're worried (except hard drives...they can have a problem with not spinning up if you let them sit forever. Take care of that with HARDWARE RAID, not motherboard RAID, and a good backup solution). Normally by the time you have hardware going wonky you can get better stuff than you have failing at a cheaper price, though.