Since the expansion valve and drier are fairly cheap I would start off with those as well. Expansion valve is less than $30 and the drier around $65. Just make sure to change all o-rings. You have to be careful when installing expansion valve. Make sure to put a lot of oil on o-rings and that it goes in straight to prevent pinching.
Another thing to check are the sensors. Your low pressure definitely works because it turned on the compressor when you added freon. However, the high pressure controls the aux fan and since your high pressure only reached 150 you really don't know if it's working. It might be safer to change it. I've seen many people bypass it (runs the fan all the time), but I prefer things to work the way they were designed to work.
I've notice that you never mentioned vacuuming. Did you guys vacuum the system for at least 30 minutes?
Are you still using r-12?