My recommendation would be to seek out a Pro seller to facilitate your order, as there’s a higher level of customer support on Fiverr’s end when it comes to Pro orders.
I am 100% sure a non-Pro seller would also be able to make a great app for you, but it is true that Pro sellers tend to be able to manage larger scale projects a bit better.
Again my recommendation is based on your initial post where you are looking for the safest option.
Now as for the order itself you need to discuss with your potential sellers and ask for a milestones based project, where payments will be released only when milestones are completed successfully.
You should also insist that the project is structured in a way that the order on Fiverr will not be completed before the actual app is delivered. So if it takes 4 months, your seller will need to make sure there are other multiple orders or the milestones cover this.