1. While you have the head(s) off to replace the HG check the block for true along with the head(s), if the block is flat and not your heads then have the head(s) milled, if both are toast then your current setup is your most cost effective and "best" option for a rebuild.
2. The easiest engine to drop in is a EJ253
3. You can use a different engine but youll have to deal with the harness merge and supporting bits not standard on the 07-09 2.5i
4. Boost. I mean at this point say your engine is toast, you "could" go ahead and drop in lower CR pistons, plumb the boost application, add all the supporting bits and tune that is going to cost a pretty penny.
5. A salvaged FA20 will cost about as much as your current car is worth.
Best option is to fix what you got first, then think about boost/turbo car/kit/etc