Can maiser money be used to purchase a Megillat Esther when one would not be purchased otherwise? If yes, may one even use more maaser for hedurim?
Maaser money can be used for a personal mitzvah if otherwise the mitzvah will not be performed.
In this case, the mitzvah of reading the Megillah will be performed even if you don’t buy a private megillah, so that maaser money should not be used for this purpose.
The most basic purpose of maaser money is for the poor.
Thus, the Maharil rules that it is forbidden to use one’s maaser money for the purpose of purchasing candles for shul. The Chasam Sofer (Yoreh De’ah 232) understands that this is because the obligation of maaser relates specifically to the poor (the Chasam Sofer writes in siman 231 that this ruling derives from the custom of giving maaser to the poor).
Yet, both the Taz (Yoreh De’ah 249:1) and the Shach (249:3) rule (citing from the Maharshal, who in turn cites from Teshuvos Rabbi Menachem) that maaser can be used for any mitzvah that a person would otherwise not perform.
However, this applies only to mitzvos that a person would otherwise not perform, and not as noted to the Megillah.