Should I marry while fearing I might not fulfill my marital duties due to masturbation, and if I grow my beard in response to a marriage condition, will I be rewarded for it, or does the saying of the Prophet, peace and blessings be upon him, "whose emigration was for worldly gain, or a woman to marry" apply to me?

Masturbation is an ugly and forbidden habit. The best way to get rid of it is through marriage. The whispers you experience about not fulfilling marital duties after marriage are from the stratagems of Satan, so seek refuge in Allah from him and hasten to marry. As for growing a beard, it is obligatory, and your intention in doing so must be to please Allah and obey the command of the Prophet, peace and blessings be upon him, so that you may be rewarded for it.