  This is the first paper of a series on the investigation of stellar
population properties and galaxy evolution of an observationally homogeneous
sample of early-type galaxies in groups, field and isolated galaxies. We
present high S/N long-slit spectroscopy of 86 nearby elliptical and S0
galaxies. 8 of them are isolated, selected according to a rigorous criterion.
This survey has the advantage of covering a larger wavelength range than
normally found in the literature, including [OIII]5007 and Halpha, both
important for emission correction. Among the 86 galaxies with S/N>15 (per
resolution element, for r_e/8 central aperture), 57 have their Hbeta-index
corrected for emission, average correction is 0.190A in Hbeta; 42 galaxies
reveal [OIII]5007 emission, of which 16 also show obvious Halpha emission. Most
of the galaxies in the sample do not show obvious signs of disturbances nor
tidal features in the morphologies, although 11 belong to the Arp catalogue of
peculiar galaxies; only 3 of them (NGC750, NGC751, NGC3226) seem to be strongly
interacting. We present the measurement of 25 central line-strength indices
calibrated to the Lick/IDS system. Kinematic information is obtained for the
sample. We analyse the line-strength index vs velocity dispersion relations for
our sample of mainly low density environment galaxies, and compare the slope of
the relations with cluster galaxies from the literature. Our main findings are
that the index-sigma_0 relations presented for low-density regions are not
significantly different from those of cluster E/S0s. The slope of the
index-sigma_0 relations does not seem to change for early-type galaxies of
different environmental densities, but the scatter of the relations seems
larger for group, field and isolated galaxies than for cluster galaxies.
