A nuc is a Permanant way of maintaining an extra queen or fifty.. Of getting extra drawn comb, and for producing extra bees to use for bolstering production hives. They can also supply the resources for more nucs if you keep them permanently, PLUS, you need to fuss with them a lot more than you should, would or will a production hive... Nucs SAVE my production hives from having me poking my nose in them as often...
But yes.. what ever box you wish to use, and advertise it as exactly what it is.. you could use a standard hive body with a follower board etc... So long as you advertise it as a medium 6 5/8 5 frame nuc, or what ever you are selling it will be fine.. I like the mediums, because even the folks who use deeps can use them, either by dropping the medium frames in a deep, or by using a medium super on top of a deep, then adding another deep under it when the first two are full.
Once I explain that the folks who want deeps usually have the light go on and they are quite content with the medium nucs.