It is appropriate to use "ni".
Yes, "ni" indicates destination while "de" indicates manner or means, as well as location, but it should be noted that ni and de can indicate location in different contexts:
Ni: Indicates long-term location.
For example Yamada-san wa ginko ni tsutomete imasu: Yamada san works in (a) bank.
This is implied as being long term, because of the nature of the job.
or: Boku wa, Igirisu ni sunde imasu : I live in England.
Again, this is implied as a long term action.
De: Indicates a brief or temporary location:
Eg. Kinou, Tokyo no resutoran de, bangohan wo tabemashita. Yesterday I ate dinner in a restaurant in Tokyo
This is suggested to be short term, as I wasn't easting dinner there for more that an hour, probably.
Or: Tosho-kan de, hon wo yonde imasu: (I'm) reading a book in the library.
Again, this is a short term action.
Oh and I think there's something grammatically wrong with you're example you used for "de". What's the construction"na~n desu"? Doesn't "na" indicate "-na" adjectives when preceeding the noun they qualify?
Anyway, hope this helps. :)