Theres nothing much to them, a rebuilt one is almost 100% brand new anyway apart from the housing and thats usually repainted also.
So its cheaper, direct fit, almost 100% brand new, and has a higher/correct output rating (less power supply issues down the track)
A non standard one is more expensive, possibly not a direct fit, 100% brand new, lower/incorrect output rating (with possible problems in future)
Weigh the benefits and make your decision. I paid approx. $130 to get an RB30 alt recond about 4-5yrs ago