Starting with FreeBSD might be easier than starting with Debian then removing all the GNUisms. But perhaps not as much Type II fun.

Using Linux gets you much more hardware compatibility especially for the consumer desktop and laptop systems this is targeted towards.

I think Linux is the better choice for replacing the entire userland. From what I've seen, the BSDs don't have such an accessible userspace/kernelspace split. With some effort, on Linux you could probably just run an exe as your init.