The RX came to the UK in Oct 2000. Anything earlier and it is an imported Toyota Harrier. As it was only available for three years there were no major changes in grades or specification, essentially the face-lift model was the one brought to the UK in late 2000. I believe there were two grades, base and SE with Nav available as an option.
A possible reason to get a series II, other than improved looks, is better fuel economy and a much better 5 speed auto transmission. The series I has a 4 speed (3 speed + O/D) transmission and whilst there have only been one or two failures reported here it seems to be a weakness of the vehicle.
Only one engine size in the UK. It is a V6 engine, also used in the Toyota Camry, fitted transverse. It isn't based on the GS/LS engine designs. As a Toyota Harrier I believe there were also a smaller 2.2l and 2.4l engines.
Yes it is a permanent 4WD with a limited-slip centre diff. This was the only option in the UK. Other markets also had just a front-wheel drive option.