"have a experienced independent overseer as the project progresses" Make sure he is from out of the area and has no connection with your contractor. Your wife's BIL probably won't do.
Ventilate the attic space well. 15 cm fiberglass insulation on the ceiling is a minimum.
Double brick walls a must. We used "Q-con-type" for the outside, red brick inside. All of the electrical wiring can be run in conduits in the empty space. If the builder needs to cut groves in the walls for running conduit have this done and completed before the walls are rendered.
Do not allow any water lines to be buried into concrete for obvious reasons. Ensure that there is an accesible crawl space under the bathrooms and kitchen for use in future repairs.
Do slump tests on the cement being used for structural supports (post and beams). Watch the mad scramble to get more bags of cement the first time you do this. Measure the size of the rebar as undersized saves the contractor money.
Use a proper septic system rather than the common cesspit. It's fine to run the grey effluent into the same tank as the black effluent. Build the tanks from concrete rings and place a pre-cast bottom rather than slopping is some cement to seal the tanks. Good idea to do a simple percolation test on your leach field especially if you are building on a former rice field.
Many other good points being made by posters in this thread. Pay close attention to the details and the big things will fall into place easily.