I agree completely with Catija's answer: no, you don't capitalize 'of', but it's better not to start sentences with numerals. However, I wanted to elaborate on when to use this 'rule', and my comment was getting rather long.
Normally, you would just write out the number, as in the 'forty-five' example.
However, as has been pointed out in comments, sometimes you don't want to write out a long number or year, like 1846. If you have a sentence that starts with such a number, like:
1846 was a very good year.
Then you can reword the sentence so that the number is not the first word:
- It was a very good year in 1846.
- A good year, 1846.
- The year 1846 was very good.
Just like any other 'rule' in English, there are bound to be exceptions. Perhaps you feel that the rewritten sentences are too awkward, and that's certainly a decision that falls within an author's creative freedom.
That being said, this rule does serve a purpose. A capital letter signals the start of a new sentence just as a punctuation mark can signal the end of the previous one. Therefore, starting each sentence with a capital letter aids readability.