Basic NanoVNA-F or H is good enough for learning, NanoVNA-F V2 if you want nice mechanical design, a bit more frequency range and better display.
Apart from that, NanoVNAs are build for low cost, low part count and tiny size, people at HP and Rohde-Schwarz did it better, 40 years ago.
Go for like an old HP-8753 if you want lab grade instrument, that doesn't do things like using square waves as test signal, that messes up most active circuits.
If they had increased budget by maybe $50-$100, the could have used something like a local oscillator/mixer design with a couple of ADF4350s in their pure spectrum range between 2.2-4.4GHz and a few switched filters to get a nice sinewave test signal istead of the nasty square waves from SI5351 and divided down ADF4350.
User interface/touch screen is implemented quite nicely, though.