So..lot of things :- )
- Longevity of SSDs is no longer an issue. Most of them have minimum "full-rewrite" of hundreds terabytes. That's just minimum guarantee, they will absolutely go much longer. They will survive longer than HDDs, don't worry about that.
- But expensive SSDs will live longer because of their higher-end NAND flash memory (SLC for corporate grade, MLC/TLC for everyday use, and QLC for budget oriented storage). Alternatively, 3D XPoint/Optane for super ultra corporate high-end.
- QLV lists are almost worthless, they barely test anything useful. You will find very few 64/128/256GB kits being tested.
128 (4x16GB) is no problem at all for 3950X, I run it like that. I bought two kits of Corsair 3200 CL16 2x32GB, and it runs super stable at 3200 CL16 at 4x32GB on Asus Strix-E X570.
- Are you sure they proposed X399 Prime and not TRX40 Prime? You need TRX40 board for 3rd gen Threadripper, not X399 which is last generation.
- Every single TRX40 board is good enough to run both Threadrippers, even Asus Prime.
- ML240 TR4 is not a good a cooler. The reason is that while it has copper plate that covers the full heatspreader of TR4 socket CPUs, internally the cold-plate (where the water touches the surface) is still under-sized because it's the same Asetek patented one as every single other on market. This cooler only pretends to be better, but is not.
There are the only TR4 designed coolers: Noctua UH14s TR4 (Excellent), Arctic Freezer 50 TR4 (Good), Wraithripper (Good), SilverArrow TR4 (Very good) and single water AIO Enermax Liqtech TR4 (Excellent cooling, very poor quality).
If any shop suggests something else, they don't know enough about Threadrippers.