Collocations Extraction using Python

Given a big text (corpus) about 1GB, I want to extract two-word, 3-word, 4-word and 5-word collocations or patterns using Log-Likelihood Ratio. More specifically, The requirements are: (1) Given the corpus, I'd like to get the bigrams, trigrams, 4-grams and 5-grams using LLR (2) Also, I want to find the collocations for any word which contains three or four specific letters. Like the collocations for words that have the letters "a - d - f" in that order but no matter if they are following one another or they are separated by other letters. In both cases, I wish to have the output sorted. And of course, as I said earlier, the corpus is 1G so it's really big. I prefer working with Python but I'm a novice so the code needs to be clear, easy to use and understand. P.S. Budget limited to $100 Thanks