Agreed, missiles are well worth the cost/resource differential over bombs.
If you can't afford to buy, there is no problem with hard building, it just takes longer. Keep in mind that the more expensive the unit, the more hammers you get for the gold! The main point is to have all your missiles available on the same turn.
Missiles are full effect (including wiping small cities off the map) for all 19 hexes. Both hits can be centered on the city, but as MrTaylordw points out, that misses the opportunity for more devastation. If I am hitting a city with multiple missiles, I like them all to be 2 hexes away, and none to be centered on the city.
Do you have the number handy? I think each hit halves the city size, but there is a minimum of 4 population lost (assuming no bomb shelter). Two nukes might not be enough. Also, rather paradoxically, wiping out a city does not generate warmonger hate like capturing a city does!
This is a choice under your control. Units within 2 hexes of a target hex will be lost. There is no RNG element.
It is not always practical to have LOS all 19 tiles, but don't nuke without good visibility (spies in the target city work well). A single unworked tile belonging to an AI or CS within 2 hexes of your target hex means that you have DOWed them.
