I second Hippo's advice about getting a TD4 instead. The TD4 can do 0-60 in 12 seconds which is very similar to your average saloon car. Sure, it's not fast but it's by no means bad. It has almost the same amount of torque as a Disco 300TDI, so you could say it has quite a lot of pulling power for a 2.0 diesel. If you want it much faster, then you should probably think twice about getting a Land Rover, or any 4x4 for that matter. Having a fast 4x4 is almost like having a fast tractor. Sure it's possible and it has been done but one could be forgiven for saying it's a silly idea.
If you want to convert a petrol to LPG, you should do it on a strong engine. The 1.8 is renowned to be a weak engine in terms of reliability. All you'd be doing is increasing the risk of catastrophic failure. Sure, there will be some who have troublefree motoring, but there will be many in the opposite position!