  We present an analysis of the distribution of structural properties for Milky
Way-mass halos in the Millennium-II Simulation (MS-II). This simulation of
structure formation within the standard LCDM cosmology contains thousands of
Milky Way-mass halos and has sufficient resolution to properly resolve many
subhalos per host. It thus provides a major improvement in the statistical
power available to explore the distribution of internal structure for halos of
this mass. In addition, the MS-II contains lower resolution versions of the
Aquarius Project halos, allowing us to compare our results to simulations of
six halos at a much higher resolution. We study the distributions of mass
assembly histories, of subhalo mass functions and accretion times, and of
merger and stripping histories for subhalos capable of impacting disks at the
centers of halos. We show that subhalo abundances are not well-described by
Poisson statistics at low mass, but rather are dominated by intrinsic scatter.
Using the masses of subhalos at infall and the abundance-matching assumption,
there is less than a 10% chance that a Milky Way halo with M_vir =10^12 M_sun
will host two galaxies as bright as the Magellanic Clouds. This probability
rises to ~25% for a halo with M_vir=2.5 x 10^12 M_sun. The statistics relevant
for disk heating are very sensitive to the mass range that is considered
relevant. Mergers with infall mass : redshift zero virial mass greater than
1:30 could well impact a central galactic disk and are a near inevitability
since z=2, whereas only half of all halos have had a merger with infall mass :
redshift zero virial mass greater than 1:10 over this same period.
