1) The word “both” is not needed because “and” means you want both conditions. Putting “both” in the example sentences is not natural. Thus, “I need to get an affordable and good computer.” Or, “I need to get an affordable, good computer." If meeting both requirements is difficult, the word “but” is useful to indicate this. “I need to get an affordable but good computer.” “I need to get a good but affordable computer.” If more emphasis is needed, then the word “both” and the qualifiers will go after the noun. “I need to get a computer that is both affordable and good.”