Bbunge is very talented at squeezing out every last ounce of performance from the hardware he is using, but what isn't directly obvious is that he is running his cameras at just a few frames per second (which is perfectly normal). So technically yes, it is possible to run 8 cameras on a quad core processor, and no other applications.
However, forum experience tells me that:
a) Many don't (want to) understand why high frame rates are just wasting hardware resources and proceed with higher frame rates anyway.
b) Many will neglect to state that, oh by the way, they were planning on using said server for other purposes.
Taking those unknowns into account, adding in some future proofing, and knowing the low price of off-lease servers on E-Bay.... if your budget is just a few hundred dollars than you will get the best bang for your buck with something that has at least 8 cores and 16GB ram.
The Proliant DL380 is one of my personal favorites. However, they moved to 2.5 inch drives beginning with G5, so unless you've got a source of cheap 2.5 SAS drives around, I would avoid the Proliants when on a budget. Also, I wouldn't get anything older than a G5 because they don't have hardware virtualization (you didn't say you needed this, but it is so popular these days you might was well get something that supports it).
I've bought several 1u and 2u servers from EBay seller Garlandcomputer over the years and have never been disappointed. Each server has arrived well packaged and in very clean condition. It was tough to tell the machines were used.
The server I am currently using as my primary is shown below. It's a Supermicro with twelve 3GHz cores (24 with HT), 24GB ram, and Twelve 3.5 drive slots, all for the price of $459.99! Can't beat that.
Here is the same chassis on sale right now for just $325:
http://www.ebay.com/itm/Supermicro-2U-S ... SwrklVJ~UE
It has less ram and a weaker cpu than the one I bought, but it will work nicely for your purposes.
If you want to go as cheap as possible, and can tolerate the higher fan speed (noise) of a 1u server, this will run you ~$175 after shipping:
http://www.ebay.com/itm/Supermicro-1U-S ... SwPhdVR9v8
To see the whole list of servers from this seller go here:
http://www.ebay.com/sch/m.html?_odkw=%2 ... r&_sacat=0
By now, you may have noticed that none of these servers come with drives, but that brings me to the final topic, ZFS.
ZFS seems to be all the rage these days, superseding the need for hardware raid controllers. When using ZFS, you actually want to export your drives as JBOD in the raid controller. At the cost of added complexity, ZFS has many advantages, one of them being compatibility with any standard, desktop-grade, hard drive. This is perfect for home users, who might have have extra WD Green or Blue drives lying around.
I haven't even scratched the surface of what ZFS can do.
Indeed, this is what I am currently running on my home server. CentOS 7 + ZFS + ZoneMinder with a combination of 8 WD Blue and Green drives (My root filesystem is on separates drives in a standard mdraid 1). The filesystem is redundant, just like hardware RAID, so if I lose a drive I can easily swap in another.
To get started on ZFS go here:
http://zfsonlinux.org/
If you are unfaimilar, there will be a learning curve, but it is worth it.