Render PC examples
As requested here are some example PC builds for iray render machines.
https://pcpartpicker.com/user/kbs666/saved/#view=7C6ZRB
This is a high end system. With a Ryzen 2700, 32 Gb of RAM, and empty slots for 32 more, a 2080ti and enough power supply for up to 2 more, which is all a North American residential circuit can support.
Depending on present pricing this system should cost roughly $3000 USD.
https://pcpartpicker.com/user/kbs666/saved/#view=YPfgwP
This is a $1500 USD build. Primarily it has a 2080. The system makes a number of compromises to make that happen but still has plenty have upgraded potential.
https://pcpartpicker.com/user/kbs666/saved/#view=q383bv
Finally is a budget build. This system comes in at around $700 USD and has 6Gb 1060. The CUP is the Ryzen 2200G which has a pretty good Radeon integrated gpu. It won't help with rendering but you'll be able to use it for you r monitor and devote the 1060 exclusively to rendering. The case is not important beyond the price, get a cheap mini tower case for this system. The other item of note is the system only had 8Gb of RAM that is the bare minimum for a functional system imo. Adding 8 more would be an obvious upgrade.

Comments
...the first one actually seems the best for the price. You could actually substitute a smaller SSD for the Boot drive (240GB) and get two 2 TB HDDs instead of the single 6 TB so you could have one for your content library and the other for backup and storage. This would save about 261$ and change on components bringing the cost down to 1,361$ and change.
I would never go below 16 GB of system memory just in case a scene dumps from the GPU which could be likely with only 6 GB of VRAM.. It would mean you would likely end up in swap mode which is even more glacially slow than the CPU and physical memory. Remember Windows and utilities like AV software will take up about 1 GB leaving you with only 7 GB for other uses. I had 12 GB in my main system (rendering on the CPU) and the process would frequently dump to swap mode when I ran out of system memory.
...interesting, as when I first oepned the links to all three, the total price on the first build was listed at 1,622$ and change (which I thought was rather low and ficured maybe they had some kind of a sale going on) . Now I see it as being 2,687$ Also the second one with the 2080 was listed at around 1,340$ not 1,524$. Only the price of the budget build is the same.
A second drive can be a backup if you disconnect it when not in use.
...as long as the HDD is not hooked into the PSU and MB it would be immune form any failure of those components. I wouldn't reccommend this for a small compact case, but a large one like I have with multiple drive bays (mine has 11) and excellent ariflow (which many protable external drives don't have) it wouldn't be an issue.
Do not count on this. I've seen PSU failures that resulted in actual fires in the case. Physical separation is a key component of backup strategies for a reason.
You can also go the upgrade path. My curent rig has been upgraded so many times the only thing left from the original build is my Corsair HX1000W PSU. Just upgraded to a Samsung 860 EVO SSD for a system drive, added a 2TB data drive to suppliment my 1TB drive & replaced my EVGA GTX 960 4GB with an EVGA GTX 1070 (all on sale at really good prices).
Current system specs:
Intel i5 4670K, Gigabyte Z97X-Gaming 7, EVGA GTX 1070, 2x 8GB Patriot Viper 3 DDR-3 1866 memory, Samsung 860 EVO 1TB SSD, Hitachi HDT721010SLA360 1TB HDD, Western Digital Blue WD20EZRZ 2TB HDD Corsair HX1000W PSU, HP DVD1720 optical drive, CoolerMaster CM 690 II Case, Samsung SyncMaster P2370 Monitor @ 1080p, Windows 10 Professional 64.
As you can see it is a hodge podge of new and old but it renders just fine so if you are on a budget, see if you can upgrade your old box before you go for a whole new one.
The point was that people keep coming in asking about buying new systems at various prices not about upgrading. While upgrading is valid to a point it does require more effort at researching component compatibility than assembling a validated new system.
Further it is functionally impossible for a person who isn't a real tech expert to do the research needed to do a system upgrade. Pcpartpicker can help some but a certain minimum knowledge is needed to use it.
..case fires are rare occasions (and the usually is the result of a cheap PSU)
The only way to go with external drives is having them in an enclosure with its own cooling system.
Drives don't need independent cooling unless they're in continuous use. A backup drive that you attach, run for a few minutes an d then disconnect should only overheat if it's own mechanical components are failing. I used a NAS as a backup system at my job for years that ran 8 HDD's plus the underlying computer on just the psu fan and the Intel stock cpu cooler. The case didn't have room for an exhaust fan.