What is the difference between a mission and a pueblo?
The difference between a Mission and pueblo is that a mission is a place for religion and also for worship. A pueblo is a place for soldiers and people to come and stay in the little village area.
It is a rumor that your LDS mission paper packet will be 'heavier' or 'thicker' if you are being called to a foreign mission rather than a stateside mission. It is true that there is extra information regarding passports, visas, etc for the specific country you are called to, however the actual weight difference between stateside and foreign mission packets is so small that it could no be measured with a standard household scale.
Perhaps not the oldest, but deserving of consideration is the Taos Pueblo which was built about 1,000 to 1,450 AD. It continues to be occupied. From a European standpoint, the Spanish Mission at Ohkay Owingeh (San Juan Pueblo) including the mission church of San Gabriel built by Franciscan friars is usually given the nod as the oldest "European" building in New Mexico.