This is really a buyer beware kind of deal.
The number of variants just in rocker arrangements is staggering......probably one for every team running them based on what I have read from guys using this stuff.
If you have a hankering to go R-5 / P-7 , you are dollars and head aches ahead by buying a complete engine......or complete heads at the least.
Then I'd want a definition of " used ". A lot of this stuff is a door stop after a 500 mile race , but a lot of it was dyno or qualifying time only.
If you do want to go this route , do a TON of homework before you get your wallet out.