So we are testing slack as our new messaging platform, it states that the platform has 10k searchable messages, does that mean that you can send unlimited messages with 10k searchable, or 10k messages total before you have to start paying?

If we are talking about the free plan: New messages can be sent unlimited, but
after the 10,000 messages limit is reached, older messages are deleted (or at least they become inaccessible) automatically.

Thanks both, if that is the case then that might be a nail in the coffin

They are not deleted. Old messages disappear from you, but if you at any point upgrade to paying plan, all old messages pop back up and are searchable and so on.

That is the only bad thing I have found about Slack, and what is holding us back from switching from Hangouts. Our sister company uses Slack, and they are used to the free plan. I am also a member on their Slack, and it sucks to send someone a message, and within a week, that message is no longer there to look back on. I find myself having to copy some conversations to txt files, or ask the other user to email it to me so I can keep track for later.

We have over 200 users in our company, we can't pay an additional per user charge per month, that would just be too much.

Try Mattermost