Empty/New 3RU and 2RU servers can be trouble to buy, ebay is usually a good idea, but be prepared to rip out the 10 year old hardware and put in a new PSU/mainboard and SATA/SAS expanders.
RU case fans are often incredible, i.e. 15,000 RPM (and higher) 40mm fans on 300W PSUs which will sound like a typhoon or 747 jet engine. if unsure, check before buying a 24 drive rack unit cheap.
UnRAID. Anyone have any experience with that?
Yeah, quite a bit really. IMO, it needs a fair amount of technical experience for troubleshooting Linux and Docker. Having used FreeNAS, Proxmox, UnRAID and other servers, it's a crash-course in IT if you want to run a windows VM on top of UnRAID.
You can run BI in a virtual machine... but if I had to mix Plex and BI, I generally would avoid the headache(s) and get a dedicated BI machine for the price of a NVR. it's within $600 or so, YMMV.
The key reason is for reliability, but also UPS power. A ~30w server and 27" monitor is easier to drive with a single 10tb HDD, than a 6-12 drive server for 5-6 hours.
Reliability is key, and i've lost drives and PSU's on servers many, many times. Having the cameras on a separate UPS and power system, along with their own battery is far more convenient in emergency situations, especially if the "cameras" UPS is also powering a monitor, along with a separate "internet" UPS for the PoE switch, wifi and router.
5-6 hours without power, sic. plan ahead.
Pro & Cons
Pro Less expensive to have a single server. Con, anything that the NAS/UnRAID needs to do, could cause the cameras to stutter or become unusable.
Pro, you can run RAID in windows with Stablebit Drivepool instead of UnRAID, con, you don't get parity or bitrot protection, and it costs money. There are cheaper drive pooling in windows 10 options out there.
Pro, It's easy to install, Con, if Windows has to apply patches, it will reboot and need to be logged in to start recording again. (this is fixable though)
Pro, you can have Intel and Nvidia transcoding, Con, Nvidia has silly encoder restrictions in their drivers unless you have the quadro P2000 series.
Pro, you can use UnRAID, con, you need to run BI in a Windows VM, and/or know how to set this up, the UnRAID server likely requires a dedicated GPU and 16gb RAM to handle UnRAID and windows 10, sic.
As for Plex, or general NAS, pick your budget and needs a few years ahead of time. It's not RAID in a conventional sense, it's a managed JBOD, which has advantages/disadvantages. Look for guides and determine the budget and what your ultimate goals are, if it's storage or flexibility, etc.
There are Linus Tech Tips videos on UnRAID and 4K NAS, just skip over the $10,000 price tag of parts, sic.
BI is ideal on something like a "desktop" SFF/NUC, a ~$200 eBay Optiplex 5060/5070 (any recent SFF PC with m.2 SSD and HDD support) with a 10-14tb WD external HDD and a ~256GB m.2 SSD.
Purple/Surveillance drives are good, but with the SSD, you can use the SSD as a write drive, with spillover/archive to the HDD and a NAS/Cloud Storage for offsite backup. With the SSD as "Hot" storage, you don't need a Purple/Surveillance model 4/6/8/10tb HDD, and can simply use a HDD that will/should spin down when not in use. try to avoid using QLC or QVO drives or cheap Intel SSD's, as the more data is written, they will lose performance. Performance models mix in high endurance SLC cache with TLC storage, that's the ideal. YMMV.
If you want to use HEVC/H.265 video from the cameras, you should spend the extra $50 and find a 2nd hand intel CPU with newer HDMI, 6th gen QSV with HEVC encoding, which is Kaby/Ice/Sky "* lake" CPUs in the last 5 years, anything above Intel i3/i5/i7 7x00 CPUs will often have m.2 slots, which allows you to mix in a 256/512 GB NVME and a "white label" WD 10tb i.e. ripped out of an external 3.5" USB enclosure for storing a week's worth locally, and then move/archive the footage using either Google Drive File Storage and a G Suite Business account for *unlimited storage, GDFS mounts as a drive letter and syncs with your G Suite Business account.
There are other cloud/backup options, GDFS is fairly practical for a Windows setup and easy to manage.
I'd also suggest creating Year/Month/Camera subfolders for archiving, as google drive tends to be slow and wieldy if you dump 100+ files into a single folder.