If you say that something will suffice, you mean it will be enough to achieve a purpose or to fulfill a need.

Is it suffice or suffice?

Both versions are correct. Only "suffice it to say" is correct. Only "suffice to say" is correct.

Can suffice be used as a verb?

verb (used without object), suf·ficed, suf·fic·ing. to be enough or adequate, as for needs, purposes, etc.