@CCruise
Mods go in Documents > My Games > Sid Meier's Civilization 5 > Mods
I have never had the VP installer install within any Steam folder.
Re #1
Upgrading is expensive. Warmongers will want to go through the Imperialism policy tree to reduce upgrade costs. Other civs should plan their upgrading around the most promoted units first. Sometimes it is better to build newer units and gift the older to city states.
Re #2 & #3
The number of technologies has an affect on your happiness. Tech too far ahead and your unhappiness will increase. Be too slow to tech and your unhappiness will increase. Religion can also play a big factor in your overall happiness too. Growing too fast increases your unhappiness. Working specialists does not produce happiness from yields as well as working tiles with those yields. This is because specialists produce their own unhappiness as well. This is why the Village improvement exists. Instead of placing farms on all your flat land you can alternate between farm and village.
I'm sure others can give more detailed information (and /or correct me) as I've only been playing VP myself off and on or a couple months.