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With 'dico' or 'loquor', a neuter adjective or pronoun is more usual than 'res' which rather means 'topic' or 'matter' in this context (aliquid de re dicere), e.g. 'plura/multa/haec/aliquid/hoc/nihil [etc.] dicere/loqui' or 'uerbum dicere/effari'...

Unum [Hoc] dicere...
Hoc solum dicere...
Unum uerbum dicere...

There is an alternative turn of phrase for 'aliquid alicui dicere uelle', which is 'aliquid aliquem uelle [adloqui]' (e.g. « Si quid ille me uelit [adloqui] » Cic.);
hence, with abl.: 'paucis [uerbis] te volo [adloqui]'.
'Unum uerbum/uno uerbo te uolo adloqui' seem unusual though.

In Terence: 'unum hoc habeo [dicere]'
So: Unum hoc tibi dicere habeo.

Lastly:
Tibi nihil aliud nisi unum dicere volo.

What do you think?

Hallo my dear chronotribe! It’s nice to read you again.

I met the phrase ‘rem dicere’ in some philosophical and rhetoric texts, but it’s not usual latinity, I know.
Look:
http://books.google.pl/books?id=rzESGVfBEWIC&pg=PA373&lpg=PA373&dq=rem+dicere&source=bl&ots=eMgaf1wPGv&sig=HD9j0QPATTIppJ47Bo2IcjWTdqg&hl=pl&ei=Vt03StXiH8mD-Ab_4am1DQ&sa=X&oi=book_result&ct=result&resnum=1

Your propositions are better, of course. I like the last version most, but I would change it a little bit:

nihil aliud nisi HOC unum TIBI dicere volo.

Is it ok?

Of course, seems better so