New to magnum swaps - opportunity came to me - is it worth it

$600 for the motor - (prices are a little higher in the bay area and there really isn't a lot of options). I didn't get a picture, the plugs were dark - I believe from the engine running rich, no build up

I paid 5 for mine with 96k on it but it looked a whole lot better than that, not that it matters what they look like necessarily.
Unless it's opened up and inspected it's a core (to the buyer) and the seller should understand that. (what if it has a spun bearing or scratched to hell cylinder?)
Personally I would tell him 200 unopened or maybe 5 if opened and looks good.
If he has a problem with that, then you will probably want to pass and look for an un altered Mag. (just my opinion obviously)
The guy I got mine from promised to give me my money back if the motor wasn't in runnable condition.
He offered to refund some of the cost just because it had cracked heads, but I told him I expected that going in and was planning to replace them anyway.

Those heads are either cast replacements (like Iron Ram, EQ) or stock Magnum heads.
EQ's say CH318A on one end with that intake bolt pattern.
If by chance they are the stock Mag heads the chances of having to swap them out for non cracked valve seat area is REALLY high.