In the event that a motor were to blow prematurely, - what generally gives out and how much would it be to rebuild? I know it could be a number of different reasons for rebuild but what is most common when the EJ goes?

Same question for Trans?

 

Rod bearings due to detonation are probably #1. Ring lands are a close second. The EJ series engines almost always [read always] require removal and a full tear down to be repaired. Proper tuning, maintenance, attention/understanding of key engine functions can help you a ton. FWIW these car would probably run forever if you keep your hands off of them. It is a turbo'd motor wear and tear are inevitable.