You'd be surprised at/by...

(The therapist is slightly dissing his clients)

Friend: Is it just a job to you, or do you care about your clients?

Therapist: ...I try to care, 'cause that makes it easier, but... you'd be surprised at/by some of the things people come to talk about. Not everything/all of it feels equally rewarding to spend your/one's time on.

1) Is the bold-face part correct and does it sound natural?

2) Would you choose 'at' or 'by'?

3) Would you choose 'everything' or 'all of it' or something else?

4) Would you choose 'your' or 'one's'?

Thank you.