File Reading and Character Storage in Data Structures

Question

I have read a text file in C#. Now I want to store the characters in a data structure so that i can delete some of the read characters inside it. My concern is,

1. Which one (Data Structure: Array, List, List<>, Linked List etc) is fast?
2. How many characters each of the types can store?
3. Is it a good practice to load so many characters in memory like this?
4. Any other idea is appreciated please.