  The evolution of low- and intermediate-mass stars on the asymptotic giant
branch (AGB) is mainly controlled by the rate at which these stars lose mass in
a stellar wind. Understanding the driving mechanism and strength of the stellar
winds of AGB stars and the processes enriching their surfaces with products of
nucleosynthesis are paramount to constraining AGB evolution and predicting the
chemical evolution of galaxies. In a previous paper we have constrained the
structure of the outflowing envelope of W Hya using spectral lines of the
$^{12}$CO molecule. Here we broaden this study by modelling an extensive set of
H$_{2}$O and $^{28}$SiO lines observed by the three instruments on board
Herschel using a state-of-the-art molecular excitation and radiative transfer
code. The oxygen isotopic ratios and the $^{28}$SiO abundance profile can be
connected to the initial stellar mass and to crucial aspects of dust formation
at the base of the stellar wind, respectively. The modelling of H$_{2}$O and
$^{28}$SiO confirms the properties of the envelope model of W Hya derived from
$^{12}$CO lines. We find an H$_2$O ortho-to-para ratio of
2.5\,$^{+2.5}_{-1.0}$, consistent with what is expected for an AGB wind. The
O$^{16}$/O$^{17}$ ratio indicates that W Hya has an initial mass of about 1.5
M$_\odot$. Although the ortho- and para-H$_{2}$O lines observed by HIFI appear
to trace gas of slightly different physical properties, a turbulence velocity
of $0.7\pm0.1$ km s$^{-1}$ fits the HIFI lines of both spin isomers and those
of $^{28}$SiO well. The ortho- and para-H$_2^{16}$O and $^{28}$SiO abundances
relative to H$_{2}$ are $(6^{+3}_{-2}) \times 10^{-4}$, $(3^{+2}_{-1}) \times
10^{-4}$, and $(3.3\pm 0.8)\times 10^{-5}$, respectively. Assuming a solar
silicon-to-carbon ratio, the $^{28}$SiO line emission model is consistent with
about one-third of the silicon atoms being locked up in dust particles.
