Eastwood does not make paint. Meaning that one of the good companies you mention makes it. Everybody has their favorite paint and method. The post is how much paint. Whether online or local purchase to eliminate waste but have a ready supply. Medium priced options were mentioned because I believe Eastwood is someone's medium priced paint and poster has decided on that price point. Trying to answer the question rather than a full blown "who has the best paint" discussion and the expletive deleted verbage. Providing he's not painting white over black, 5 quarts should be enough.

It's backwards, but leaving the inside last means that you can wait for more paint to arrive and a slight mismatch to the exterior is impossible to detect. Painting inside and outside same color makes the backward OK.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn