Lost Mines of Phandelver as PC base [on hold]

I am DMing Lost Mines of Phandelver, and it’s looking like the party wants to keep the cave as a base to replace the BBEG and turn evil.

Main question is: if I decide to let them fix up the forge of spells after a few quests for parts, what level / how challenging should this be?

I love the rule of unintended consequences, but am not sure of any down sides.