The only contemporaneous source we currently have that explicitly bears witness to the existence of the Therapeutae is Philo's "De Vita Contemplative" (The Contemplative Life), which is dedicated to discussing the Theraputae. The Therapeutae were an Egyptian Jewish sect that lived a secluded monk-like life near Lake Mariout in the vicinity of Alexandria.
Rabbi Ze'ev Ya'avetz in Toldot Yisrael, vol. 4, pg. 150-151 writes about the Therapeutae and translated their name as "כת בני התרופה", "the cult or sect of the sons of the medicine", "as they heal their souls from guilt and blame."
Here's the list of customs they had mentioned by Rabbi Ya'avetz, based on Philo's work:
They lived a secluded life, the men didn't marry women and the women didn't marry men, they lived in poor clay houses far from the civilization, they wore only cloth tunics, they would eat only after sunset and even then, they abstained from wine and meat and only ate bread with salt (though the more pampered also ate some moss), the youth drank cold water and the elderly hot water. Most of them lived on a small hill close to Alexandria. They would purify themselves and keep away from all impurities, they did a lot of charity work, each had a private prayer chamber and the only things allowed in there were holy scriptures and they would contemplate there all day long the teachings of the Torah and prophecy and in the books of their ancient sages, and they would also write songs of praise to Hashem. When they ate, they would discuss Torah and when one answered another's question, they would all clap and cheer and one sage would sing a song of praise and they would all join in, the men by themselves and the women by themselves, and they would sing until sunrise, upon which they would once again separate to contemplate. On Shabbat they would all come together at the synagogue (with separation for the men and women) and one of the elders would talk about Torah. And some of them would fast for three or even six days.
The only relation I see here to Nazirites is their abstinence from wine.
Rabbi Bernard Revel in the Karaite Halakah also wrote about them:
"The tenth century Karaite, Abu Yusuf al-Kirkisani, in his work Kitab al-anwar wal-marakib (written 937), speaks of a Jewish Sect named "the Magarites" (אלמגאריה). This sect, says Kirkisani, sprang up before the rise of Christianity. The adherents of the sect make the biblical passages that speak of the attributes of God refer to an angel who, according to them, created the world (ed. Harkavy, 304). Among them are the works of the "Alexandrine" (אלכסאנדראני) which are the best of the "books of the Cave" (ib., 283). The same author, speaking of Benjamin Nahawendin whom he considers the second founder of Karaism, says that Benjamin's belief that an angel created the world is similar to the view held by the Alexandrine (ib., 314). Harkavy ingeniously suggested that these "Magarites" are the Egyptian Essenes, known as the Therapeutae. The "Alexandrine" whose works they so highly estimated is no other than Philo (ib., 256 ff.) and Nahawedni's "Angel" goes back to Philo's "logos"." (pg. 87)
Therefore, according to Rabbi Revel, who quotes Albert Harkavy, we can add a gnostic-like theology (i.e. that God created the world through a subordinate but powerful entity that possibly also runs the world) to the Therapeutae, their holding Philo in high esteem, and also another name, the Magarites or Magharians. This idea is also mentioned by Rabbi Ya'avetz here.
