go mode "S" now
If you are puting the package together now you would be unwise not to fit a mode "S" unit as the instalation cost is a big part of the total cost of getting mode "S".
Some King units just a "box change" from a mode A/C transponder but these will also require a bit of programing if you fit a replacment unit as if you do not have the new rack fitted you cant have the E-prom that carries the aircrafts individual data.
Narco tell me that they are looking into a mode "S" unit that will fit into the AT50/AT150/AT155 rack but they have no in service date yet for this unit.