Looks like the engine from Importmotor includes rebuilt heads.  This
probably keeps the price way down.  Since head failures are the common death
of wasserboxers, I have been told to avoid rebuilt heads, as they might have
previous corrosion problems, but then again they might not, so unless you
were present during rebuild, you might be taking a risk, low price no doubt,
but more risk than a higher priced engine with new heads (all other things
equal)
  I suggest you ask more questions of them if you want to buy a rebuild from
them, also check out other rebuilders, ask what they include.  Read some of
the Boston Bob articles on rebuilding heads, rods and so forth.  Also
consider is the engine properly balanced, and is the compression ratio set
right.

  It is easy to throw parts together and get a motor to run, it is another
thing to correctly rebuild, replacing parts that are less than perfect, but
then the motor last longer (but costs more)

  This may  be a good deal, but ask and maybe check the better business
bureal to see if they have any complaints or not.

   good luck

dave