MGB overdrive

["Ron Fine" <RonFineEsq@earthlink.net>]

I am going to pull my engine and transmission from my 66 MGB this summer for
a complete rebuild.  I am considering installing an overdrive which it does
not currently have.  From time to time I see overdrive units for sale on
Ebay, etc.  I was wondering what I should be looking for when buying a used
overdrive unit and what is a  reasonable costs of such a unit?
Any help is always appreciated.
Ron Fine
66 MGB
61 BN7