"Need to" as an English Modal

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)

• You need to talk to Bill. ~ You don't need to talk to Bill.

• *You need talk to Bill. - You need not talk to Bill.

• He dares to protest ~ He doesn't dare (to) protest.

• *He dare protest ~ He dare not protest.

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.