The actual coders decided that we would like to start with a basic design and get the functionality there.

As a coder myself, I'd say that it's a mistake to not get that design done before making something that gets seen by the community (what the coders make for testing and never gets seen is immaterial).

My development uses the MVC pattern, so the actual look doesn't matter too much, but I can build the functionality without knowing what the UI looks like.

I guess you guys need to know what tabs/sections will be required and what options can be changed. The coders can build that functionality and the UI can be made to reflect how the user interacts against that functionality.

Get the design done properly. Reduce complexity by only implementing one tab/section/.., then build on it.

If you don't finalise the UI and functionality first, it will turn into a lot of grief to change it afterwards.

Better start with something you're proud of from the get-go.

It's not a race, is it?