what portion of sedan interior will fit avant?

front seats, center console front dash and doors and I think that might be
 

it, I know reaer sedan and avant seats are different, not sure on the rear doors though.

i think the rear door shells are the same (window frame different), also i've been told that
 

the sedan seats will fit into an avant and vice versa with some slight modifications to the latching area.

do you mean rear's?

yes, rears. it's what i've heard. i haven't seen it done, though.

Rear seat backs are avant specific. The rear seat bottoms are the same.