Magnum, Magnum R/Ts

I have a few questions regarding some heads.

1.) Is it a good idea to buy used Magnum or R/T heads?
2.) Where to buy?
3.) Average cost for the used pair?
4.) Same question as the above except the goodies (with rockers, rods, valves etc)

I'd rather not buy new if I didn't have too.
Thanks for looking 8)