Why do you want a Discovery at all?  If you live on the motorway, they
are not the greatest things, but it all depends on what you drive on
when you get off the motorway.

I've done many thousands of miles in my previous V8i.  LPG is a good
thing to have, but get big tanks.  I had two 35 litres tanks, and they
don't get you much further than 200 miles.  I used to fill up every
day, which means that on balance a diesel would be much faster if you
knock the 10-15 minute filling time and general inconvenience off the
trip.

With LPG is used to get 31 mpg equivalent.  Petrol has since gone up
in relation to LPG, so it might now do slightly better.  It took a few
goes to get the LPG dead right, but worth it when it is.  

Upgrade the brakes and get the shock absorbers changed if they aren't
new.  Hit the brakes hard at 70mph + on a damp motorway - you'll see
what I mean.  I ran the tyres at 32 psi front and 36 rear, not the
half-flat settings that Land Rover recommend.