How to proceed with funding and getting a prototype?

Should we get angels to come in after we have made a prototype or should we wait until we have an established base of customers? We realize that we can't go to investors without a prototype to show off. The problem with waiting is that our idea is a platform that requires a lot of initial investment for it to be worthwhile for consumers and our business clients. We have the money for the initial basic prototype but beyond that will require quite a bit of money to build the platform.

For the basic prototype, we plan on just having the fundamental features on an iOS app. But since we are a platform, should we have more to show? For example, should we also have the cloud based software for the business clients we will partner with? Should that be sort of functional or is that not needed in this stage of the game?

If you have a really good proof of concept you can easily articulate in a sentence or two and can somehow demonstrate the capacity to make it happen, you're in better shape than most. The prototyping should happen after the idea has been developed and workshopped with unforgiving scrutiny. If the idea is bulletproof you're in good shape.

I think any visuals or slides that help you drive home the idea and the value position are worth putting together ahead of actual product prototypes.

... since we are a platform, should we have more to show?

... should
we also have the cloud based software for the business clients we will
partner with?

... should that be sort of functional or is that not
needed in this stage of the game?

You must ensure that from your prototype and presentation you clients/investors/partners will get the whole idea, explain the problem you are solving and how your product is solving it.

You don't need to show the smallest details of how the product works. You must show why works, and why is possible and viable.