I doubt you'd need the full Lanman or MS-Net client for DOS (those suck enough conventional memory on their own without adding an NFS client on top!), you probably just need the underlying TCP/IP stack and NDIS driver for whatever NIC you're using if even that. I would have thought the PC-NFS client would include all of the required bits out of the box rather than requiring you to buy a competing server product first, but if not installing the MS-net client and stripping out the unneeded bits may be the easiest way to go. I imagine the end result is still going to be rather conventional memory-hungry compared to something like the NetWare or EtherDFS client though.

You may also require a PC-NFS compatible authentication server on your network for it to work properly (at least this was the case with the Beame & Whitesite NFS client and, I get the impression, many other DOS NFS clients). NetBSD still includes the Sun PC-NFS authentication server which should work if this is the case.

Actually, the memory requirements of those clients is an issue that I have been trying to fix for a bit.
Other memory managers can move the client modules out of conventional memory, but anything other than memmaker makes Virtualbox a bit unstable, well thats my experience anyway.

Actually, I wonder if anyone has some in-depth docs on MS-NET?
Particularly what each module does.
Or is it buried somewhere in Developer docs?

There is PC-NFS docs on internet Archive.
https://archive.org/details/pc-nfsuserman