FR - I think I’ve seen every airhead variant used as a basis for a bike in this thread. Most of those would’ve come with an 18” rear, stock. It only gets tricky when you start with a paralever if you want an 18” wheel.
It would seem that a twin shock model might give you a greater range of options for longer suspension in the rear, but it’s do-able regardless. Just a matter of what you’ve got to start with and what parts you can get your hands on with the budget you have.
Find a bike and start from there. Also bear in mind that not everything you see here will function as well as it looks. That said, airheads have fairly long travel suspension as stock bikes, and people have been “scrambling” them since well before someone decided to call it that.