I "think" the answer to the first question might be in "descr_cultures"

from the file;
port_sea data/models_strat/residences/roman_port_03_sea.CAS
watchtower data/models_strat/residences/watchtower_roman.CAS, watchtower_roman
watchtower_cost 200
spy spy.tga spy_info.tga spy.tga 350 0 1


as you can see here they set the model and cost for watchtowers, tough its on a per culture basis and I doubt you can change them per faction (than again I'm not even sure the above entry is actually correct)


G