I've built and sold two businesses for £1M+ each. In both cases I took on outside investors.

Some things to think about:

Do you need an investor? What would you actually do with the money he’s investing? You mention an office and adding video production, but are those things you really need to have / do? What is really constraining your growth right now? Would having an office and more money actually generate more sales? You mention that you already have two commission-only salespeople, so they only cost you money if they bring in sales, so you wouldn’t need more money to get sales through that route.

If you proposed partner is into flower wholesale and distribution, they may not really be able to add that much useful expertise to your social media marketing agency.

I’d look at it this way: what’s the worst thing that would happen if you did or did not take up this offer? If you did take it up, worst case is that you lose your business and can’t do anything about it (things don’t go well, partner stops funding, business goes bust); if you turn it down, you are at least still in control of your own destiny.