You're in a tough spot but if you're going to replace the engine, make sure you deal with a yard that will take the engine back for whatever reason. I've seen plenty of "great deal" motors come and look terrible because they were from a fire car, flood car or some other crap car. If they're willing to send it to a shop and take it back, it means you probably won't have an issue with it because at that point, they don't want to eat what margin they make on an engine sending out another one.

Also, shop rate for something like this is around 10 hours depending on what needs to be swapped between the two engines so unless your mechanic is moonlighting, $600 sounds pretty cheap and very rushed... which translates to possible problems later on down the road. Another thing that may not be taken into account is the cooling system needs to be thoroughly flushed.

Another thing is that what ever caused the issue needs to be determined or you may be doing this a second time. Since it was in an accident, it may have overheated before it was repaired but you should really make sure that is what happened and not that there is some other reason.

If you were in the Chicago area, I have a couple very low mileage engines that were pulled from cars for V8 swaps and I'd be happy to do the labor as well but I think you're far enough away that it wouldn't make sense and even shipping could be an issue.

Take care