Not to worry. Front loaders do NOT WALK. Hell, they hardly move at all
- the forces are entirely different.

Having said that, think of how a wood floor system is built - maybe 2
x 8 with a three-quarter inch sub-floor and tiled over and build that
and trim out to match your cabinets. Don't over think it, Yes the
washers are HEAVY, but the kid from our local Sears actually lifted it
out to the truck for us! Boy, is he going to have back issues later on
in life!

Get the pair - take your time - look for closeouts. We got the RED one
for $39 (washer) and saw the Dryer go or $199! (Originally $1,100
each!