There is no external evidence for the Buddhist texts before Asoka. The evidence for Asoka's time is extremely limited - a few texts mentioned by name with no content. All dates in early Indian history are highly conjectural. There is for example no other evidence than Buddhist texts for if or when the Buddha lived, and that evidence has produced a number of different dates. A symposium on the dates of the Buddha in 1991 concluded that he lived ca 480-400 BCE. In 1992 Richard Gombrich independently reasoned from the texts a very similar date - see Dating the Buddha A Red Herring Revealed. These are the currently accepted dates amongst most scholars. It looks like Wikipedia is drawing on the traditional Theravādin date, which is only accepted within the Theravāda sect and no longer accepted by non-sectarian scholars.
The dates of Asoka are reasonably certain, since four Bactrian kings are mentioned in the edicts and these can be aligned with physical evidence of these kings (such as coins) in Bactria. So we know that Asoka flourished in the mid-3rd century BCE (ca. 250 BCE). On Asoka and his dates, see Charles Allen's excellent book Asoka. Similarly, in the texts the Magadhan Empire has yet to expand beyond Magadha, and it's future capital at Patna (Trad. Pataliputta) is still a small frontier town of little significance. Suggesting a time before the Moryan Dynasty founded by Asoka's grandfather Candragupta - so probably at least a century before Asoka.
Less precise, but of considerable importance is the beginning of the second urbanisation in the Ganges Valley ca. 7th century BCE. Evidence for this is extensive, for example, in the form of the ruins of walled cities at Rajgir and Shravasti (See Romila Thapar's Ancient Indian Social History: Some Interpretations).
In the texts the Magadhan Empire is not yet an empire and Pataliputta is unimportant. Early Buddhist texts do not mention Asoka, so we assume that they predate him, at least by a century, ca 350 BCE. The texts do mention well established walled cities so we assume that the antedate them. If these assumptions are valid (and not every scholar agrees that they are) this gives us a range of ca. 600-350 for the composition of the Buddhist texts. This is at least not inconsistent with most of the dates derived from the texts, which place the Buddha somewhere in the middle of this range.
The texts show considerable evidence of having composed orally and existing as a widely dispersed oral literature for a considerable time before being collected and written down. See for example Anālayo's article Oral Dimensions of Pāli Discourses: Pericopes, other Mnemonic Techniques and the Oral Performance Context. This is consistent with the Sri Lankan chronicle Mahāvaṃsa which asserts that the texts were not written down in Sri Lanka until about the first century BCE during the reign of King Vaṭṭagāmini. Writing texts down in India is less certain, but probably began around the same time.
The earliest physical evidence of early Buddhist texts are birch bark manuscripts from what is now Pakistan and Afghanistan (ancient Gandhāra). One or two texts are carbon14 dated to the 1st century BCE, and a few more to the first two centuries CE. The oldest Pali text is a small manuscript written on gold from the 5th or 5th century CE.