How large of a farm did you have in mind?
If you go for few machines but more powerful ones, then you probably will pay a price premium for the CPU to get a lot of cores (you will spend beyond the sweet-spot for price/performance ratio) but instead fewer machines will take up less space and you will need fewer motherboards, PSUs, cases etc.
I would go for AMD ThreadRipper systems running for instance AMD TR 2950X (16C/32T), 2970WX(24C/48T) and 2990WX(32C/64T). Beefier cooling will also be required to tame such beasts att 100% load and perhaps some OC. The more threads your CPU will run, the more RAM you need. Like at least 1GB/core. CEP2 project that ran on WCG earlier had huge RAM requirements for instance. Also, get a good motherboard with decent VRMs so that they wont wear out fast due to high temperature.

If space and such is not of a concern, then you might want to go for sweet-spot price/performance per machine and many machines instead. I would probably go for AMD Ryzen 7 2700 or 3700X (both 8C/16T). 2700 is of previous generation but very nicely priced right now and has cheaper motherboards. 3700X is more expensive, but has higher performance per core and the new motherboards are more expensive due to PCIe 4.0 implementation in the x570 boards. however, it is possible to install a 3000-series CPU in an x470 motherboard (cheaper), but you need to have a correct newer BIOS version for it to boot with those newer CPUs. So either the motherboards of choice must have something like BIOS-flashback to be able to flash a new BIOS without CPU installed or it must come with a newer BIOS at delivery, or you must have a 2000-series CPU that is compatible with the board at an older BIOS version to then be able to flash all the boards you buy to a newer version and then swap the CPU for the 3000 series one.

Good value for money right now would be something like:
AMD Ryzen 7 2700 + and OC (or 2700X if you do not want to apply OC yourself)

Asus ROG Strix B450-F Gaming (Decent VRM for this price class)

16GB RAM @ CL15-16 3000Mhz are pretty cheap (2x8GB for Dual Channel preferred)

Pretty much any HDD/SSD >= 120GB for OS install, BOINC and its data directory.

Good PSU is extremely important. Choose a Corsair, Seasonic, BeQuiet, CoolerMaster and such brands with 80PLUS GOLD efficiency rating at >= 500W if you are going to run only CPU tasks. If you are going to run CPU and GPU under load at the same time choose >= 750W. Better ratings than GOLD exist (Platina, Titanium for very high efficiency ratings which will help lower the electricity cost but the up-front investment make them pass the point of diminishing returns).

If you are not going to run GPU projects, or will invest in GPUs once projects will become available here at WCG then right now you can buy cheap passively cooled GPUs just to be able to connect to a monitor if need be. On the other hand, if you are going to do GPU work outside of WCG then you must check if those tasks are CUDA or OpenCL to find the best GPU for you.

I would choose an air-cooler for such a system since they have much higher reliability than water coolers. Either one can go cheap with some Hyper 212 cooler or go fancy with a Noctua-cooler and Noctua fans to get the best of the best in terms of quality, reliability, noise, temperature, OC-ability.

Finally, chassis. Good air-flow is very important for 100% load 24/7 systems, make sure there are air filters for intake fan positions to minimize dust build-up. A bonus if there is a fan int the front, positioned so that it blows cold air from outside the chassis right into the CPU cooler. Other than that, is doesn't have to be anything fancy.

Hope this helps! :)

Thank you for your kind words, I am happy if it can help people! :)

I would agree with the things mentioned in the post before.
You can find an article about the crunchers I currently run here:
https://dc.sheridon.de/detailed-specs-of-my-crunchers
I described in detail which components I chose and why.

They were build before the 3rd Generation Ryzen was available, but I think the Ryzen 2700 is still a very good choice. Ryzen 3700X or 3900X might be the best choices if you want to spend the extra money (and gain the extra energy efficiency) of the 3rd gen Ryzen.

Thank you to all my fellow crunchers for the fabulous advice.
I hope to begin my build soon
Build is complete: see below for results

if you can find cheap X399 boards, look for cheap Threadrippers... i got a 1950X (16c32t) for $450 last black friday (and an 8c16t 1700X, too). the 1950X is fast enough to get top 1% in Sheep-it (rendering farm)... even like top 14 or so, and that was even with missing a few days due to a stupid client-banning thing that most people run into

just be aware, there's a chance X399 may be dead for Zen 2 Threadrippers, tho i hope not.

if you don't care about CPU time, the AM4 Ryzen 3900X for $500 does a bit better (and uses less power and generates less heat) on 12c24t, and is faster than a 1950X in Blender, with the 16c32t 3950X coming in November. you can get a nice hearty board for those for $90-190 (B450-A Pro + air flow or X570 TUF Gaming for example)

the only point of X570 is PCI-E 4.0. i'm mainly running nice hearty MSI B450s. X399 starts at $250 new, tho i scored one for $185