A lot of it will depend on what age of hardware you are looking for, especially the price. Last year I upgraded all my machines to Poweredge R620 servers. These are old enough that you can find a lot of options for CPUs and memory dirt cheap, and you can find them with either 2.5" or 3.5" internal hot-swap bays. The 6xx series is 1U and the 7xx series is 2U chassis. If power is a concern, I run some VM servers with around 10 VMs in use, 64GB of memory, and a pair of 12-core Xeon E5-2630L v2 processors (2.4GHz, low power) at around 84W, but there's plenty of options to customize to your needs unless you need current-generation horsepower. The PERC controller in them can be flashed to IT mode for full control, and I run ZFS through it for some of my machines. I built these machines for around $150-200 each and picked them up from ebay (there's a US seller I can recommend if you're interested in going that route).
Keep in mind the R* series are rack servers, but Dell also has tower versions of the same machines available -- I think those are labeled as M620?