The 4.2 is, in my opinion, one of the best reel to reel decks available. You’re getting sound quality equivalent of 1/2" Stereo in that format. I have several and they are beasts and I love them.
One thing to be mindful of with the 4.2 is that usually these were in use by film or video houses and have miles and miles of tape dragged across the heads. You can get the heads replaced though so it isn’t a big worry. But if you can see the thing ahead of time and know how to check the heads it’s worth knowing. Not a deal killer–you can get them replaced.
Also you will want to know which audio curve is built in, again not a dealkiller (both curves are great) but something to know.
You will also need a collection of cables and adaptors to fit it into a contemporary music production universe–the line out is banana jack, unbalanced. The line in (if you are using your own preamps) is a specific DIN connection. If you are using the Nagra preamps (which are nice enough honestly) check which setup it has as there a zillion variants with and without phantom etc etc.
If there was a Nagra 4.2 near me at a reasonable price and the mechanical functions were more or less in working order I would not hesitate to buy it. Mono is a fantastic format and worth exploring.
I don’t know where you are located but if in the US, Trew Audio in Nashville has done service work on my Nagra collection and always been prompt, professional, and done good work. They can also get you the cables you need for interfacing it with contemporary audio production gear if you don’t want to make your own cables.