That's where Dave and I differ, I wouldn't do the iron block unless you're planning on 800+ horsepower or tons of boost. I like the weight savings.
You can get an LS2, or LS3/L92/L99 at damn near any decent recycling yard.
You don't really need different heads (cause they aint cheap) unless you've got spare cash and are shooting for those 650hp (motor) numbers. Heck, even the aftermarket manifold could run you 700-1000 depending on which one you pick. And an aftermarket throttle-body is a few hundred as well. It adds up big quickly if you are aiming for 580+ on motor. (there's always a small nitrous shot I guess)
don't forget ancillary stuff like motor mounts, tranny mounts maybe, proper speedo conversion, ECU + reprogramming, headers for LS ain't cheap either. Plus the oil-pan Dave mentioned. Man, it's like throwing dozens of Benjamins on things after the motor purchase. (speaking from personal experience)
You can get 475-550 (motor) horsepower easily with stock heads/intake and a good cam/exhaust selection. (and knock down 23-26mpg on the highway with an OD)
personally though, I like seeing a big block in an Impala rather than an LS motor. Maybe throw one of those FAST auto-learn fuel injection units on it and hide it with an open element air cleaner,....pop for some aluminum heads if the $$ is there. Drive-ability and torque with old school cool.