I’d recommend going with a good freelancer (if you can manage him/her yourself) or a normal agency (if you want someone to manage the designer for you). In any case I would think more about the deliverables you need instead of the amount of hours / iterations you need in the beginning. When you see that you have enough work to hire a designer full time I’d go that route then, as it is (usually) still cheaper than such an offer (IMHO).
Stay away from sites like upwork.com if possible (the quality is not good in my experience) and prefer people in your own city if possible, as being able to meet face to face is a great advantage in my opinion and helps to build trust. Do not hire the cheapest designer available but also don’t overpay, ask for cost estimates and references. Make sure you’ll be able to work with the design they do for you and be sure to make it clear who is responsible for implementing it or in what format the results will be provided (e.g. Sketch or Illustrator).