Path of Exile free to play in about six hours.

[LochnarITB]

You've thought a lot more about this than I have so I won't be very much help with most of your questions. My planning has been pretty much "cold witch", "do I have a slot for that cool gem?" and "does stuff die?". For the tree, I generally save a handful of points and then try to find a line that points toward some special node. I focus on damage then mana and then life, with a little dex and str for gear. For equipment, I do check vendors for pieces, especially upgrading jewelry, but it has mostly been stuff I pick up that gives me better boost and still has the needed slots and links for my gems. Otherwise I leave any white items and vendor any unused blues and up to turn into orbs to buy anything I do come across. I have not tried trading with others but I never did in other games either so I probably won't.