New Testament Documents

I am beginning my studies on the New Testament. I know that the 4of
them were written somewhere between 40 and 100 a.D. I am curiousif
the original scripts of this documents still exist or all the
information we have are based upon post-written copies of
the original texts.
If so what it's the aproximate date of the oldest's known copies.
Also I want to know if this copies can be considered to be trustable
or there is the possibility of some modifications by the Church during
the first century.

Jose Rubens Ambrosio
University of Brasilia