i would NOT use the onboard water pump to dump OLD WATER.
any crud IN that water may get sucked up into the system and clog lines OR the pump.
drain via the tank drain values and perhaps use a basin/pale to catch some of the water as it exits...
for visual inspection.
one can also run a soft hose into the FILL tube and down into the tank and siphon OUT water this way.
sanitizing the system is easy, follow the cdc/nih guides or the proportions offered IN the owners manual or on the A/S website.
MORE organic stuff IN the water means higher concentrations or longer exposures are needed to sanitize.
IF the tank has been sitting with the SAME WATER in it for years, there is a reasonable chance that surface algae has developed.
this stuff is mostly harmless but can still muck up the system AND the idea of stuff growing ON the water is yucky...
one can use a flexible fiber optic scope to LOOK into the tank for stuff, these can be rented now.
and there are inexpensive water testing gadgets now that provide info about HOW MUCH stuff in in the water...
water FILTERS and FILTRATION is an entirely different subject with many many threads here...
once the tank is useable and full o'fresh water and the external water line is connected AND drainage is established...
run the water and pump and ALL of the fixtures a bunch while INSPECTING all of the connections and pipes...
it's not uncommon for minor DRIPS to be found that are solved by hand tightening connectors.
leaks not solved this way need more attention, don't ignore them.
yes to all of the water heater suggestions above too, and don't forget to CLEAN the PRE/screen for the water pump.