fundamental to
Kiuhnm
Please have a look at the following sentences:
1. They lacked something fundamental to be truly happy.
2. They lacked something fundamental to being truly happy.
I think they're both acceptable as the "to" in the first sentence can be replaced with "in order to" and the one in the second sentence is a preposition associated with "fundamental".
What do you think and which one comes more naturally to you?
EDIT: On second thought, the second one is better. "In order to" suggests something deliberate to reach a certain goal, I think, so it's not appropriate in this context. That doesn't mean that "to be" doesn't work, though.