the M62 powered 4.4 V8 L322 on LPG is a fine conversion, the M62 converts well.
Expect 14-15mpg as a good average and mix of driving and roads.
The 02 - 05 models with the ZF 5HP24 box are prone to torque converter issues and also an O ring failure between clutch packs that can cause issues.
Box refurb can set you back around £100 to do yourself or "2500 for a specialist to do.
The 06 onwards model year with the newer ZF boxes are no where near as prone to GB issues, but the Jag AJV8 engine doesn't convert to gas so well....with tales of valve seat regression due to the lack of hardened valve seats. Some have fitted a Flashlube system which dribbles oil into the intake manifold to keep the valve seats lubed and cool, but the number of tales told of knackered engines would lead me to suggest it doesn't convert well....not to say it can't be done!
The ride in the L322 is firmer....for almost a year I had both the L322 and a 4.0 P38, the ride in the P38 was less harsh and the seats are comfier I have to admit. But the power on tap with the M62 engine, the more taught steering, better cornering and handling characteristics of the L322 make it a more relaxed drive and I wouln't swap it for nothing....
When the girlfriend was pregnant and after the baby arrived, she struggled to fit in her Fiat 500 so I handed her the keys to the L322 and I bought myself another E38 7'er (had 4 or 5 previously) which was fun and comfortable, a real urban brusier and motorway bruiser.....but it wasn't a Range Rover, so I got the P38, which I was very happy with, but after curing and oil leak, I struggled to get reliable oil pressure despite a series of tasks...bearings, front cover, oil pump etc....so It was passed on to pastures new.
We got the girlfriend a Ninety pick up which she adores, and I got my L322 back....and I have to say, there is nothing like it!
I love Range Rovers and I would love to have one of each - a Classic for show, a P38 to play with and the L322 for best!
But of the 3, I'd pick an L322 any day of the week....yes they are overly complex, they are expensive to maintain and run, they are tempermental....just search my threads for one on diagnostic system failure....a Xenon HID lamp blew and wrecked havoc with the car, and for the best part of a year I have not been able to connect using diagnostics...I tried all sorts of new parts, tests, pulling fuses, hard resets etc...but all of sudden for no rhyme nor reason it started to communicate again...unless the headlamps are on....even if I disconnect the headlamps completely....go figure.
They do have a large back end, and in the snow and ice, you do 'feel' it, like a large counterweight just waiting to act like a pendulm, but awesome to drive, and I wouldn't swap it, unless it was for another L322 (2012 I think is the best looking L322)