the cheapest most reliable and I guess safest would definitely be the instax as most have said.
but if you'd rather get a Polaroid and go for the TIP, then you could get a polaroid 3000RF instead of an sx-70.
it's an all plastic body with rangefinder, rather than zone or fixed focus like the other plastic body polaroids.
it also uses integral film (100, not 600) so would work with TIP film.
you can usually get them for under $30.
if you'd like, I have one I can send over to you, just pm me.