Historicity of Siddharta Gautama and the early history of Buddhism?

Because he lived so long ago, there is no written evidence of his existence from his time, but most historians consider him a historical figure from his influence. There isn't actually any body of Buddhist texts from the estimated dates he lived, as the invention of writing on palm leaves did not happen until centuries later. Thus, the early teachings of the first 18 sub-schools were actually memorized and passed down orally, and are full of pneumonic devices like lists and repetition to achieve retention.

The earliest students also did not care for making images of his likeness, maybe similar to Muslims with Mohammed. The first statues were actually made by Greco-Buddhists in what are now, ironically, Muslim countries.

As far as origin, everything grows out of something. Most of the basic concepts already existed in ancient Indian religious culture. The idea of caste and having a 'dharma' or duty to fulfill your role, before entering old age to finally pursue spiritual duty was in place. There were also life-long ascetics and the idea of samsara, the cycle of rebirth, and moksha, liberation.

A lot of the cosmology was inherited as Buddhism did not negate the existence of hindu patheon of heavens and gods, but just placed them within the cycle of existence and not outside of it. Its more that the attitude changed towards existing ideas, rather than completely rejecting the culture.

Another influence was caste, as Shakyamuni belonged to the Kshyatria, warrior caste and most of his students were from his clan. There is a militaristic vibe and language such as referring to those that achieved liberation as arhats, "foe-destroyers". Early monks shaved their heads, donned the uniform of robes and had proscribed discipline which became known as the vinaya. It's very soldier like. The Buddha didn't just lay out the rules, but would make them on a case-by-case basis as problems developed in the community. There are also warfare-related analogies in the dhammapada, the sayings of the Buddha, to where the path is likened to battle, but interally against the passions. All of which would have been very well-suited to his mostly Kshyatria audience.

Later on, there were lots of Brahmin converts who were very well-educated and liked rituals and scholarship, somewhat the reverse of the earliest wave of teachings, at which point many sutras appear with vast, cosmic stories and infinite numbers of Buddhas. This became known as the Mahayana, but occurred some centuries after the first period.

The earliest texts are in Pali, a form of Prakrit, the vernacular language of Kshyatrias, but then with the Brahmin converts of the Mahayana period, Buddhist text began to appear in hybrid Sanksrit, the formal language of Brahmins. It's not proper Sanskrit, but Sanskritized Prakrit, to where you can see the Brahmin influence on the linguistic level, let alone for the vastly different concepts. There isn't just one period of Buddhist religion, but like Christianity, there are various periods that evolved along with Indian culture.