If historians do not know much about Muhammad the man, can you believe anything you read about his life on the internet?

Wikipedia pages on Islam are almost uniformly awful. The internet being what it is, most of the pages seem to have been edited and written either by the devout (that is, believing Muslims) or by people who seem to be either interested in atheism or religious criticism but from a solely Christian background. So you end up with an entire page on "The Historicity of Muhammad" as though that was a similar question or controversy as the historicity of Jesus on the one hand, and on the other you have an article on "Hadith studies" where Hadith criticism gets barely a passing mention.

It's a bad joke, and I would suggest avoiding Wikipedia on these questions as much as possible.

To return to your original question:

If historians do not know much about Muhammad the man, can you believe anything you read about his life on the internet?

It might be worth laying out why historians can't claim to know much about the man. At first blush, it looks like we have lots of sources. Chief among these are:

(1.) The Qur'an. Muhammad's revelation. Unfortunately for our purposes the Qur'an is an esoteric document. It is not an equivalent to the gospels in having qualities resembling a historical account. When it does refer to historical events it does so obliquely. So for instance in one of the rare examples in which the Qur'an makes mention of a contemporary proper noun (there are only a handful of such examples) it explains the Battle of Badr like so:

Allah had already given you the victory at Badr, when ye were contemptible. So observe your duty to Allah in order that ye may be thankful.

And that's it. That's the whole description of the battle.

For an explanation of what even these few references mean we have to turn to:

(2.) The Tafsir. The exegesis of the Qur'an. These are sort of the "explanatory notes" that take obscure passages and record what is thought to be there meaning. There is no single definitive exegesis of the Qur'an, and there are any number of passages where the exegetes were as stumped as we are today.

(3.) The Sira. The biography of the Prophet Muhammad, as well as the literature associated with his campaigns. The most extensive/definitive biography that we have is ibn Hisham, who edited and added to the sira of ibn Ishaq, whose text is lost. Muhammad died in 632, ibn Hisham died 200 years later.

(4.) The Hadith. The collected sayings and recorded practices of the Prophet Muhammad. The key feature of the hadith is that although they were recorded long after Muhammad's death, each hadith report includes a chain of transmission going back to a personal companion of the prophet. So a typical hadith, in full, looks like this:

Qutaiba reports, that Ismail bin Jaffar reports, on the authority of Abu Suhail, on the authority of his father, on the authority of Abu Huraira—may God be pleased with him (NB: this indicates in the report that Abu Huraira was a companion of the prophet)— that the Prophet Muhammad—may God's blessing and peace be upon him—said: "When Ramadan begins, the gates of Paradise are opened."

Note that the English version omits this chain of transmission, though it's included in the Arabic. Critically in order to designate these chains of transmission as "sound" or not, the scholars who compiled these hadiths tracked the lives and relationships of the transmitters. So for each of those names above you can consult enormous biographical dictionaries that describe the reliability of purported transmitters. These reports don't have to be uniform, nor were they entirely expected to be. In Muslim ibn al-Hajjaj's collection, for instance, he'll typically include as many as a half dozen variant hadiths, some more detailed, some less, or the same hadith as recorded by a different chain of transmitters.

All of this material is incredibly extensive. Taken together, the Qur'an, a standard tafsir like Tabari's, the Sira of ibn Ishaq/ibn Hisham, and the six "sound" hadith collections would make up many many thousands of pages. That seeming abundance of material led the 19th century French semiticist/orientalist Ernest Renan to claim that in contrast with Christianity that "Islam was born in the full light of history."

The problem is that the early material (namely the Qur'an) has very little detail. And the detailed material (e.g. the Hadith), is recorded 200 years later.

Starting at the beginning of the 20th century it became clear to Western academics, starting at least as early as Ignaz Goldziher at the turn of the century, that much of this material was based on later fabrications by 8th and 9th century legal scholars. So, to give just one example of a kind of forgery, we have writings from Islamic religious and legal scholars citing a report that's recorded as being the comment of a companion of the prophet, but then in the next century that same report is attributed to the prophet himself. This led Joseph Schacht, for instance, to conclude that:

every legal tradition from the Prophet, until the contrary is proved, must be taken not as an authentic or essentially authentic, even if slightly obscured, statement valid for his time or the time of his Companions, but as the fictitious expression of a legal doctrine formulated at a later date.

The next generation of scholars, looking at early Islamic sources more generally, including the Qur'an itself, went further still. Patricia Crone's work is largely based on the notion that none of this material is early enough or reliable enough to reconstruct the history of the 7th and 8th centuries and so instead turned to non-Islamic but contemporary sources. Her book "Hagarism," though widely rejected today, particularly given subsequent discoveries of early Qur'an manuscripts and a number of inscription-based evidence, set the field on its head.

I could go into greater detail, but the fact is that the problem has not at all been solved. I would strongly encourage you to read Robert Hoyland's essay Writing the Biography of the Prophet Muhammad: Problems and Solutions for a more in-depth view of the historiographical problems involved here.

The second half of this issue of course is that none of this has any bearing whatsoever on the practices of believing Muslims. The Qur'an and the Hadith are two of the bases of law for all Muslims. While the Sira and Tafsir don't quite have the same canonical status, they might as well for many believers. Ahmad ibn Hanbal's notion that tafsir and the biographical literature of the prophet have no foundation would be a very strange one to most contemporary Muslims.

In that sense, your question in regards to ISIS, for example, is really an epistemological one. Contemporary historians might not "know" much about Muhammad because they are critical of the sources. Believing Muslims who put their faith in the reliability of the Hadith can "know" the Prophet Muhammad down to minute, almost obscene levels of detail. We "know", for instance, what the Prophet had to say about defecating, and periods, and how he dyed his beard.

So yes, in that sense if you want to "know" what devout Muslims know about the prophet, there are plenty of good resources. As I've linked to, I very much like https://sunnah.com/. For the https://quran.com/ very nicely provides multiple translations, I prefer Pickthall. I can't easily point you to a good translation of a classical Tafsir, but there's a huge variety of modern ones. I don't think there's a free online edition of the biography of the prophet. The most widely available English edition is Alfred Guillaume's translation The Life of Muhammad. If your interest is specifically in ISIS and how they use this material, all of ISIS' official magazine publications are available here. Be warned that some of the material in these magazines is extremely graphic.