GM Crankshaft & Harmonic Balancer Advice

They say 4 year warranty ???? You mean they will brake after 30 k or the warranty will expire after 30 k?
I think he's saying they'll crater around 30k. You can probably invoke the warranty-- at great time or expense OR BOTH to you -- but a steaming pile replaced with a steaming pile is still......

....a steaming pile :D
If you had a head gasket problem, or a crank problem, I might say repair it. But both problems will cost more than a good used engine.
I'd find an engine with 50-75k miles, maybe do intake gaskets before dropping it in the car. (Common GM 3.1 and 3.4 failure)
You will be money ahead, and end up with a better engine.
I did a head gasket job on a 3.1 and when I got it all apart found one of the heads was cracked, so there is no guarantee it wont get more expensive than your current estimate.