As the title states, looking for motherboard, CPU, RAM, PSU.
Not sure if this is understood, but I would go for an 80 plus or 80 plus bronze PSU are the norm for any cruncher. I would have said titanium, but the extra <10% efficiency will barely cover the +$100 cost. I have an Antec earthgreen 380w for my primary 24/7 cruncher. Saves me a lot of cash in the long run compared to my old non 80 plus psu setups. It is also built to run 24/7 for years. Not sure on the durability of systems that will be turned on and off at least 1 time a day. Since this is a dedicated cruncher, that PSU would be perfect with any Ryzen chip and a low to mid end video card:
From newegg:
https://www.newegg.com/Product/Product.aspx?Item=N82E16817371033
For CPU, I would recommend getting the 1600 or 1700. The extra benchmark increase of the 1700 over the 1600 is minimal compared to the electricity rise and for crunching, electricity cost is everything. This is also due to the 1600 being clocked slightly higher than the 1700. I personally was debating between the 2, but for the 4 thread drop, you are losing around 300 points on benchmark according to passmark. For me it isn't worth the extra $100. The X versions will consume much more electricity and only give a tiny bit more performance. If you are after PPD, you are better off with the X and/or 1700 series. If you are looking for power efficiency, you are better off with the non X. If you are looking for runtime and have the extra $100, I would go 1700.
RAM and Mobo I would have to pass and let other Techs answer.
EDIT: I meant electricity use of the Xs compared to non X's. The 1600 and 1700 are both rated at a TDP of 65w and although AMD's TDP does not reflect actual watt usage, they are both still pretty on par with each other.
The benchmarks of the 1600 and 1700 are still only 300 points off on passmark though. And that is a little jump for $100.