More things I've dug up recently:
"Zen Buddhism... [is] a natural evolution of Buddhism under Taoist influences."
-- D.T. Suzuki
"Dr. Daisetz Suzuki (D.T. Suzuki), the equally eminent authority of Zen Buddhism, speaks of it (Zen Buddhism) as a natural evolution of Buddhism under Taoist conditions."
-- Dwight Goddard, "History of Ch'an Buddhism previous to the times of Hui-neng (Wie-lang)" (2007) In: A Buddhist Bible, Forgotten Books (publisher)
Highly referenced and citated section on the Wikipedia.org page on Zen Buddhism:
"Taoist terminology was used to express Buddhist doctrines in the oldest translations of Buddhist texts, a practice termed ko-i, "matching the concepts", while the emerging Chinese Buddhism had to compete with Taoism and Confucianism.
The first Buddhist recruits in China were Taoists. Against this background, especially the Taoist concept of naturalness seemed to have been inherited by the early Chán disciples: they equated—to some extent—the ineffable Tao and Buddha-nature, and thus, rather than feeling bound to the abstract "wisdom of the sūtras", emphasized Buddha-nature to be found in "everyday" human life, just as the Tao.
In addition to Taoist ideas, also Neo-Taoist concepts were taken over in Chinese Buddhism. Concepts such as "T’i -yung" (Essence and Function) and "Li-shih" (Noumenon and Phenomenon) were first taken over by Hua-yen Buddhism, which consequently influenced Chán deeply. On the other hand, Taoists at first misunderstood sunyata to be akin to the Taoist non-being."