I have a lenovo P52, it came with 16 GB ECC. The dimm is not super-expensive, but it's hard to find, and only comes in half the size of regular dimms.

I replaced it with non-ECC memory and never had an issue. In fact you can combine ECC and non-ECC memory without any problem.

Yep, you can get it on the higher end trims of the P-series Thinkpads.