well 6wheelers are out there.. lots actually
even eight wheelers are not uncommon either...
both are -more often than not- amphibious
the full individual suspension.. IDK... but I don't see why not
"kart-sized" Hovercraft, were available by mailorder in germany in the late eighties.
Can't remember the make or model, but I remember browsing the catalogue.. popping my eyes and nagging my parents to buy it as my birthday present... until I realized that "price on request" never is a good sign...
So at least your not the first crazy enough for such ideas
'sid