Outsourcing is a great way to get your app faster without giving up ownership. Keep in mind the relationship likely won't end when you launch though - you'll want to plan for subsequent releases, as well as rapid optimization sprints as you hone in on P/M fit. You'll want to make sure you're comfortable working with that firm for an extended period of time. Don't spend your whole budget on the initial release.
It's also worth hiring a friend or respected dev to do a code review of any potential vendors. A common problem is outsourcing v1, bringing in a technical lead or cofounder, and finding out your code is in disarray and needs to be rebuilt. To the degree your app is leveraging native iOS elements that risk might be mitigated, and the disposition of the person you're bringing in is obviously a variable (some devs have "not invented here" mentality). But the peace of mind is worth the couple hours you'll pay for a capable code review up front.