Well my wife's 2002 Explorer XLT 2x4 4.0L with 197K has the rattling coming from rear of the motor and has been diagnosed as worn rear chains/guides

Dealer wants $3000...Pfff not gonna happen

Indy gave me 2 choices:
1. Parts (chain/guide kit) & labor for $850 but he said he's done it before and it still had the noise so he said he can't guarantee the problem will be gone.

2. He'll source a lower mile used motor and swap it in for $450 + cost of motor.

they both seem like a gamble so I am turning to you guys with experience, which way would you go? Bare in mind I know most will say to just get rid of it, but with the kids in private school we can't afford another car note until my Audi is payed off in 2 years.