It is accepted. "Un plan" is used for a map to a city (plan de la ville) or a street map (plan de ville). "Un plan de métro" is a map for the metro system. "Une carte" is used to refer to a map of a country, region, or perhaps a highway system.
"Sa" is possessive adjective (used before a noun). It may mean either "his" or "her". "Elle" may be a subject pronoun or an object pronoun. Open this link in a browser for a complete explanation: http://french.about.com/od/grammar/a/pronouns.htm
No. A possessive adjective is used like this: sa pomme = his/her apple, son livre = his/her book. A possessive adjective is only used with a noun. "Hers" is a possessive pronoun, e.g., Cette pomme est la sienne = that apple is his/hers.