Everything works on context, so I would have no preference without context. If it's just a sentence with no context, you can choose whatever you like. "I have prepared myself..." is grammatically correct.
I think Copyright gave you the right answer, the first time. All of the sentences that you wrote are grammatically correct, and they would work in certain situations. If you want to use the sentence with a more active voice, then you should say 'I am prepared'. Although 'I have prepared' is present tense, it uses the past participle, and it says that you did something in the past. It suggests that you made preparations in the past, and you will not do anything else to prepare yourself.
If you really think an employer will be carefully reading your reply letter, then you want to avoid sounding like you've already done all that you plan to do, in order to prepare for their training.