sounds like you need to get that car to a place that's experienced in running down CEL issues. probably NOT an "out-in-the-driveway at 18 degrees" kind of fix.
you can subscribe to a good online repair manual for any year and model subaru at alldata.com, it's like $30 per year. without detailed schematics and parts lists on hand, you will NEVER get rid of those pesky codes....hell, you'll probly never even find the parts involved!
the P1507 idle air control may be a vac hose issue, or more likely need to have IAC valve/sender replaced. IAC is a black valve ass'y on side of throttle body.
P1592 neutral position switch may be due to short shifter, the switch in question is one of the two electrical leads on pass side of gearbox...NOT the speed sensor one. check to be sure the sensor wire isn't damaged before digging into the switch itself.
P0328 knock senser IS a (relatively) easy fix...just make sure you get the new one installed in the same orientation as the original...like 30-40 degrees to the center line of engine. You might want to check the orientation of the one in there now...if it's loose and has rotated off its axis, that could trigger the code.
this is the kind of stuff that would drive you NUTS on a swap. you have an only mildly modded non-turbo car....swappage would likely entail a 10x increase in this type of problem down the line.
