first, lets distinguish between the three parts of this challenge and the two messages in the first part. Therefore, we are talking right now about solving message 1 and message 2 from part 1.
From message 1 you can learn the key which was used for the transposition. You started correctly by blindly stripping away the Caesar.
Now, the hint in challenge tells you, that in both messages, the same transposition was used. Hence with the knowledge of the transposition-key, you can remove the transposition from message 2. What remains is only the Autokey, which should not be hard to solve.
As for solving the transposition cipher.. well the hint is there: it has a key length of 13 and it is ciphertext-only, thus nothing is known. However, nowadays a key length of 13 isn't beyond reach of todays computers. Additionally it is pretty easy with a more sophisticated ciphertext-only attack on transposition ciphers. Thus, this transposition was not meant to be attacked in the same way as the transposition used in the molehunt series. Additionally, language skills (i.e. german) are not really required for finding this solution. Therefore, maybe another hint would be that although this is a level I challenge, you probably need a computer, but the algorithms are out there..
The hints for solving part 2/3 (ie. the ADFGVX-challenge) are given in the decrypted cipher-text ….and text-structure.
Happy decrypting :)