Is this combo worth it over a cheaper CPU and better board?

If multithreaded work pays your bills, the 9950X is the bigger gain. If you mostly game, a mid-range board with a cheaper CPU gives better balance and more ports.

Will the B850M-K handle the 9950X at full load?

It runs, but the VRM is the weak link under sustained all-core loads. Use a well-ventilated case and a top-down or tower cooler that moves air over the VRM.

What cooler does the 9950X need?

A strong 240mm or 360mm liquid cooler or a large dual-tower air cooler. The chip ships without a cooler and reaches its power limit under heavy loads.

Does it support PCIe 5.0 and DDR5?

Yes. The board has a PCIe 5.0 x16 slot for the graphics card and DDR5 DIMM slots with EXPO support. Check the manual for which M.2 slot runs at PCIe 5.0.

The ASUS PRIME B850M-K board and the Ryzen 9 9950X CPU. No cooler, no memory and no storage are included, so budget for those separately.

Can I upgrade to a future AM5 CPU?

AMD has committed to the AM5 socket beyond this generation, and B850 boards support the current Ryzen 9000 line. Check ASUS's CPU support list before buying a later chip.