if you want info about LPG I rather suspect that the best places for that would be some of the UK based Yank car sites or possibly some of the Landy forums. I seem to recall there was a black one for sale about 4 months ago which had been LPG converted so obviously it can be done but I can't remember any details IIRC a system will set you back up to £2k - that buys a LOT of petrol before it starts to make it financially viable

as for values usual suspects - pistonheads/ebay/autotrader.