Land Rover Defender Steering problems

Ive just bought my first Land rover (defender 90 TDI 1991), and just
wanted to check a couple of things.

First off, it has Power steering, but this only works correctly when
the engine is rev'ed a bit, is this normal, or symtomatic of a problem
with the sterring pump?

Also there is about 10% of play in the steering, is this acceptable, I
traced the problem to the steering box, if I did replace it, would I
notice a significant improvment (as I get the impression that if you
get a replacment steering box, its actually a reconditioned one)

Thanks for any info

Cheers Simon