Don't forget that you stand a good chance of finding a manual/overdrive gearbox at the spares day coming up in October.
If you get the correct (original) type of gearbox in your car then you will probably be very happy with it. I don't think anybody's ever said anything bad about the Jaguar 4 speed. It might not be as slick as the modern gearboxes but it is of its time and suits the character of the car. I also believe that it is repairable if anything should go wrong with it.
Once you've got your gearbox you can then embark on the debate about the correct oil to use in it.
1962 E-Type FHC
1969 420 Daimler Sovereign
1994 X300 XJR