I have quite a few featherweights and teach classes about them. So, I do have my favorite things about different machines from different years. I like the throat plate that has numbers on it so something later than 1953 would usually have that.
I want a quiet machine. Sometimes that's hard to determine if it hasn't been cleaned, oiled, greased and the motor lubed. Doing all of that will always give a quieter sound.
I have a white machine but it doesn't have motor ports. I would encourage people to only buy white ones with motor ports. Without those you can't really service the motor.
Cosmetic condition is usually what people look for the most. It doesn't have to be mirror finish but I don't want major issues.
I own 13 machines now and always look for more.