"From my knowledge, the original texts are nowhere to be found. So, how much of that can you really trust? How much of what is in the Tanakh (Old Testament) can you be 100% sure was there from the beginning?"
For one example, you have different manuscript families that represent the Hebrew Old Testament.
You have the Hebrew text of the Hebrew Old Testament which is found both among the Jews and among the Samaritans.
The Jews have their textual family of the Hebrew Old Testament and the Samaritans have their textual tradition -- of the five books of Moses.
These two traditions, textual traditions of the Hebrew Bible, have been separate for thousands of years.
The separation of the Israelites in general from the Samaritan Israelites is recorded all the way back in the Book of Kings.
Now, despite that these two manuscript traditions being separate for so long, if you take a look at the differences between the Samaritan version of the Torah and the version as preserved among the Jews, you'll see that while there are very many differences, essentially all of these differences are differences in spelling - variant spellings.
This is similar to the variation of spelling the word C-O-L-O-R in American English and the word C-O-L-O-U-R in British English.
I'm not so up-to-date, so it might have just been in the King James Version of the Bible.
There are certainly other words, honour, or center.
In America we say C-E-N-T-E-R and in Britain they spell it C-E-N-T-R-E.
So, this is the bulk of the differences between the Samaritan version of the Torah and the Jewish version.
There are a few significant differences, and I mean really a few.
One of them is, I believe, in the Ten Commandments, where the mountain regarded as holy by the Samaritan is referred to there by name, where as in the Jewish version no mountain is referred to by name.
It simply says "the place that God would choose," or something along those lines.
There is one other place, where it refers back to some occurrence that is recorded in both the Samaritan and the Jewish version of the Torah.
In the Samaritan version it refers back to it verbatim, word for word, as opposed to just referring back to the occurrence in general.
Now, there are a few lone places, a handful of places, that there is an actual word difference between the Samaritan text of the Torah, the Jewish text, and the text as found among Dead Sea Scrolls, and the Septuagint (the Greek translation of the Torah).
These handful of places where there is an actual word difference do not change the overall meaning of the verse, nor do these ever regard the commandment to do or to abstain from doing any particular act.
So these differences have no impact on fulfilling the actual commandments of God.