Should I rebuild a "good engine" to fix oil leak?

Try to find where the oil leak is coming from first before you dump all that time and money into a full rebuild. Oil can leak from the valve covers, oil pan, timing cover, or intake manifold (usually that one affects performance though). You could also check the oil pressure sender at the back of the block. FYI a bad rear main seal will cause only a couple drops after the car is parked and turned off.