This sentence of the landlord can be understood as a whole or literally. But from a grammatical point of view, it is wrong, because rest assured is a verb, which can be used with preposition object or object clause. Therefore, the landlord can use this sentence orally, but it is not recommended to use it in formal written occasions.

If I turn it over, my personal suggestion is:

Choosing us means choosing. Don't worry.

Or you chose us, and you chose the guarantee. (Note: the past tense is used here to emphasize the result, that is, "I chose peace of mind").

If you help the landlord, please adopt ~ ~