thats always the gamble you take...
buying statside you will have all sorts of iffs and buts - even buying from a UK supplier there may be some hassle, as the dispute in event of a non functioning trans is often thrown between the installer and supplier.. what happens then, you have spent a grand or more,, and still dont have a working car..
with someone like GM transmitions its simple... take the car in, 3 days later hand over cash and drive out.... no hassle no worries and if it comes grinding to a halt a week later, its fixed again for free