  Context. CARMENES is a stabilised, high-resolution, double-channel
spectrograph at the 3.5 m Calar Alto telescope. It is optimally designed for
radial-velocity surveys of M dwarfs with potentially habitable Earth-mass
planets. Aims. We prepare a list of the brightest, single M dwarfs in each
spectral subtype observable from the northern hemisphere, from which we will
select the best planet-hunting targets for CARMENES. Methods. In this first
paper on the preparation of our input catalogue, we compiled a large amount of
public data and collected low-resolution optical spectroscopy with CAFOS at the
2.2 m Calar Alto telescope for 753 stars. We derived accurate spectral types
using a dense grid of standard stars, a double least-squares minimisation
technique, and 31 spectral indices previously defined by other authors.
Additionally, we quantified surface gravity, metallicity, and chromospheric
activity for all the stars in our sample. Results. We calculated spectral types
for all 753 stars, of which 305 are new and 448 are revised. We measured
pseudo-equivalent widths of Halpha for all the stars in our sample, concluded
that chromospheric activity does not affect spectral typing from our indices,
and tabulated 49 stars that had been reported to be young stars in open
clusters, moving groups, and stellar associations. Of the 753 stars, two are
new subdwarf candidates, three are T Tauri stars, 25 are giants, 44 are K
dwarfs, and 679 are M dwarfs. Many of the 261 investigated dwarfs in the range
M4.0-8.0 V are among the brightest stars known in their spectral subtype.
Conclusions. This collection of low-resolution spectroscopic data serves as a
candidate target list for the CARMENES survey and can be highly valuable for
other radial-velocity surveys of M dwarfs and for studies of cool dwarfs in the
solar neighbourhood.
