Also, how exactly do you achieve the undervolt? Would you do a voltage offset of like -0.1v in the BIOS, so that all voltages run at 0.1v less than auto? Or is there some other preferred method? Any advice appreciated.
AFAIK you don't need a -K SKU to undervolt it, but you do need a motherboard which allow a negative offset to Vcore (as the ASUS usually do, but it's just an example).
The BIOS (UEFI) method is more trouble-proof than any sw-based method, at least under Windows (like the old RM Clock), so I would go for it.
About the various quoted CPUs, providing you won't overclock, I would pick the cheapest I could find.