what are your power goals? this will determine what I would personally be looking at as far as turbo, injectors, pump etc upgrades. As for the cooler, pull the exchanger out and run the new cooler setup standalone on the box
my guess is the low 300wkw side with said mods @ 12psi
irrelevant
do it all at once, not when things go wrong. If you can budget it, do it before it puts a hole in the block and certainly not after
as above, if you have a power goal in mind, that makes these questions much easier to answer