Depends on the sector, category if its B2B or B2C, normally in a B2B space there will be always an existence of multiple players and market will be fragmented. B2C space is where you need to be a little careful & ensure you move fast enough and have a differentiation with a plan to build that.

Security is challenge in wordpress if you are using anything from the market. Most of the themes & plugins do get the job done by at the end of it also leave your website vulnerable to attacks. I would recommend using custom developed website for such purpose & get a 3rd party security testing done before you go live so that you ensure atleast the regular known attacks are taken care of.

You should have a solid, non-poaching, non-compete & NDA with the agency you are trying to collaborate with. I believe it all boils down to trust at the end of the day :)