The modality of need and dare (sometimes called "semi-modals") is a matter of Negative Polarity. Their meaning is modal, but their syntax is variable.
Inside the scope of a negative, they can be used as modals; outside it, they can't.
(Sentences with an asterisk in front are ungrammatical)
As to whether need to "is a modal", that depends on
- whether you allow anything with a modal meaning, like maybe or have to, to be modals
- whether you allow verbs with to, like have to, need to, dare to, and want to to be modals
- whether you allow verbs that inflect for tense (like dared to or needs to) to be modals
and probably a few other things.
All modals are complicated, each modal is unique, and there are simply no general rules about them.