I've completed the game and I have found the Sultan just twice. I did a full RS conversion, well, as close to it as you can (as Rockstar left out the short exhaust, wide arches and coupe chassis). Then the game f*cked me over and abra-kadabra'd it into thin air.
So when I do find one, it will eventually be lost, without a doubt because the impound feature isn't very good.
The question is then, why the f*ck are there so many Sentinels and Rapid GTs roaming the roads and not any of the fan-favourites?