  The largest uncertainty for cosmological studies using clusters of galaxies
is introduced by our limited knowledge of the statistics of galaxy cluster
structure, and of the scaling relations between observables and cluster mass.
To improve on this situation we have started an XMM-Newton Large Programme for
the in-depth study of a representative sample of 33 galaxy clusters, selected
in the redshift range z=0.055 to 0.183 from the REFLEX Cluster Survey, having
X-ray luminosities above 0.4 X 10^44 h_70^-2 erg s^-1 in the 0.1 - 2.4 keV
band. This paper introduces the sample, compiles properties of the clusters,
and provides detailed information on the sample selection function. We describe
the selection of a nearby galaxy cluster sample that makes optimal use of the
XMM-Newton field-of-view, and provides nearly homogeneous X-ray luminosity
coverage for the full range from poor clusters to the most massive objects in
the Universe. For the clusters in the sample, X-ray fluxes are derived and
compared to the previously obtained fluxes from the ROSAT All-Sky Survey. We
find that the fluxes and the flux errors have been reliably determined in the
ROSAT All-Sky Survey analysis used for the REFLEX Survey. We use the sample
selection function documented in detail in this paper to determine the X-ray
luminosity function, and compare it with the luminosity function of the entire
REFLEX sample. We also discuss morphological peculiarities of some of the
sample members. The sample and some of the background data given in this
introductory paper will be important for the application of these data in the
detailed studies of cluster structure, to appear in forthcoming publications.
