Since you already have a radiator, you could always save yourself some grief by taking it to a readiator specialist & ask him to strip it & give you a condition report.
Often just a good clean out will restore a radiator to service quite inexpensively, & if the core is US, then you have no guarantee that another second hand one is any better.
Ai the old core is beyond saving, then a new replacement core is my recomendation instead of a second hand unit in equally unknown condition.