That makes MD5 a very poor choice for a hash post, as you could conceivably attack it by finding an incriminating statement that has the same hash, forcing the person to reveal their true statement.
I think SHA-256 should be safe, at least for the next couple of years.
Not really. You can find a statement that has the same hash, but no guarantee you can find an incriminating statement that has the same hash – it might very well be “H4f#¤TFDadffgd”
Hash collision is a problem for security*, not for secrecy.
*I will happily elaborate how it is a problem if you want it