Prices will vary a lot depending on what mileage is OK with you.
However, finding one that will have either the integrated boosters or the rear facing boot seats could be near impossible. Let alone both...
Parkers only list the rear facing seat as an option for models between May 2000 and Sept 2000! If that's true, I've no idea why it was withdrawn so quickly...
See also:
http://www.volvoforums.org.uk/showth...ght=rear+seats
V70 is fine with most car seats, but the rear seatbelts are a little short (intentional design feature) and the headrests can't be moved, so some seats can't be fitted.