The most cost effective approach to Lunar exploration that is cumulative, builds on itself over time, is to begin by sending a nuclear powered tunneling robot.
Not a terribly high tech device, and rather slow and methodical, even if it just dug an inch of tunnel a day, in a year the tunnel would be over 30 feet long. At intervals, just like in an Earth hard rock tunnel, it would drill thin holes and insert reinforcing rod to stabilize the ceiling. Likewise, it would spray sealant on the bare rock against micro-fissures.
Because it is on a one way mission, its lander could be cannibalized for pressure doors, flooring, ceiling and walls. These could have modular conduits for wiring, so that the tunnel would have electrical heat and power adapters available.
The tunneling robot would probably mine a much more elaborate tunnel than a single shaft. Branching shafts, both horizontal and vertical, could have extra space for things like a water cistern, so that harvested and purified ice could be kept liquid in quantity; a greenhouse, of sorts, to produce supplemental food and purify the air somewhat, maybe even producing ethanol for fuel; and additional storage for breathing air tanks.
Finally there would be a niche for the reactor, so it could power the habitat when the astronauts arrived.
Having robots do the work prior to arrival of humans would save an enormous amount of time and money, and the humans could bring far more supplies and equipment with them, extending their mission and allowing them to conduct much more detail operations at much lower risk.
The tunneling robot would probably mine a much more elaborate tunnel than a single shaft. Branching shafts, both horizontal and vertical, could have extra space for things like a water cistern, so that harvested and purified ice could be kept liquid in quantity. . . . Purity Of Essence.
Finally there would be a niche for the reactor, so it could power the habitat when the astronauts arrived. Having robots do the work prior to arrival of humans would save an enormous amount of time and money, and the humans could bring far more supplies and equipment with them, extending their mission and allowing them to conduct much more detail operations at much lower risk.
"I would guess that, ah, dwelling space for several hundred thousands of our people could easily be provided. A computer could be set and programmed to accept factors from youth, health, sexual fertility, intelligence, and a cross section of necessary skills. Naturally, they would breed prodigiously, eh? There would be much time and little to do. The prevailing emotion will be one of nostalgia for those left behind, combined with a spirit of bold curiosity for the adventure ahead!"
