To my mind the best strategy is to first purchase new sources of food, so to buy level 2 of farming. Then I buy the level 2 of science to get tech points faster and level 2 of medicine to be sure that my villagers will not become elderly too much early. After that, I may buy the level 2 of building but sometime I prefer to buy the level 3 of science, then level 3 of farming and level 3 of medicine. Here I'm quiet for doing all the other things to do!!

(You can get everything from hints to spoilers in the Virtual Villagers Guide .)