Correct, you can only buy a power seat and install. Usually there is a couple plugs for ground and battery on the main plug.
Word of caution, if the H3 has SWS (seat weight sensor) or SPS (seat position sensor) detection in the seats, it can affect airbag deployment (I think they do because my 06 manual seats are connected to a body harness). Also, on new model seats to not cut or damage any yellow connectors. You might get a surprise.
It has both switches. What you call a seat weight sensor is the sensor in the passenger seat that determines a full size person is sitting there, so that will turn on the passenger air bag. No one in the seat means the passenger side airbag will not deploy in case of a frontal collision.
the seat position switch signals the SDM the seat position. If the seat is at a certain distance from the air bag, and the SDM estimates a stage two air bag deployment is required, it will deploy at stage two. If the switch shows the seat is close to the bag (meaning the driver is inches away from the bag) it will only deploy blow the bag at stage one. (Stage one is a less force full blow.)
And as noted by MTUH3, if the wire ends in a yellow connector; make sure to pull the air bag fuse. Yellow connectors denote the supplemental inflatable restraint system and that includes air bags, sensor, side air curtains, SDM (aka the black box) and the pretensioner system built into the front shoulder harnesses.