NADM51 whit Ifi usb purifier or ifi usb converter

I don't know, but suspect you will get a lot more first-hand feedback at computeraudiophile.com.

I read the M51 benefitted a lot from an Audiophilleo feeding it. I have an AP with PurePower and can't recommended enough. But it is more expensive than the iFi alternatives.

I have also read that the M51 benefitted from outboard USB/SPDIF conversion. I believe the ifi purifier is an additional upgrade to the ifi converter.

I have no experience with the nad dac but have used both the usb iPower and the usb-spif converter, the iLink, to my dac. The iPower splits the usb power from the audio data with a double-headed usb cable. I find both essential in improving sound quality from the pc.

I have the combo of iPower with the Gemini cable feeding to the iDAC in my library system, w/o the iPower and Gemini cable the sounds is ok, with the combo the iDAC is pretty good. You may want to look at the AQVOX usb power supply.

All computers, even run in battery mode, generate too much garbage on their DC lines, including USB. There are some DACs that are less prone to that, those where the USB-chip is powered from the DAC internally. IMO the NAD M51 is not.