C4 Grand Picasso is a great car, no idea of 2nd hand prices, biggest boot of all the 'occasional' 7 seat MPVs, handles well, far better than the zafira (although the way the back end of a zafira breaks away can be fun!) when back seats are down the boot space is absolutely enormous.(They fold completely flat, unlike zafira's which push up against the backs of the front seats)
Zafira is a poor car IMO, poor finish, poor handling and all the styling of a milk float. I've currently got a C4 Grand Picasso, had Zafira, 307SW and FRV for over two years each, Zafira easily the worst car of the bunch.
FRV is a good car and again handles well. The boot is pretty small with the back seats up, although you can fold the seats down individually, the three abreast seats are good as one kid can sit in the front with you, a good behavior modifier as only good kids get to sit in the front!
I carry a lot of stuff for work and what I had to pack carefully in the back of the FRV just disappears into the back of the C4 with no effort.
307SW is probaly the cheapest to buy out of the cars I can comment on, and is an OK car, again better than a Zafira, similar boot area, but you have the downside of Peugeot reliability.