I converted an older (E-Series) window van and recommend you think through all the implications associated with starting out with a wagon. There are a lot of differences to consider.
I used a wagon to make it feel more open (visually) and it caused a lot of problems, plus it limits what you can build. I often regret not using a cargo van. And with newer Transit differences are even greater.
I'm not suggesting you don't use a wagon, just that you think about all the differences in advance. It's not just about extra seats.