Get the stag kit from Jeremy at u-fit-lpg. His ebay user is u-fit (I think) but there are links to Jeremy's details on here somewhere (I'll have a look in a moment!)
You will pay under £700 for the full kit if you go for the more expensive 4-hole tank (80Litres) which isn't essential on the 3.0. I have, but only because I do a lot of miles (25-30k) and a lot of towing! When I collected my front end kit I was talking about multi-vales, as opposed to 4 separate valves, and there are apparently some new ones becoming available soon which can allegedly cope with up to approx 275bhb, so are a viable option for all Miggys!
As for the fitting... As you are only in Reading, Kevin Wood may be able to advise and possibly even lend a bit of a hand (obviously I can't say this for sure because I don't know what his time and commitments are like!!) I can almost certainly lend you a spare inlet manifold so you can install the injector nozzles in advance (on an exchange basis!)
I wouldn't worry too much about the mileage, as long as the car has been well maintained you should get at least the same again from it!! Mine's just gone past 121k and runs like new!! Also, LPG being a cleaner fuel means that the engine internals will last longer!
I think there's going to be a full "how to" made up very soon, with input from all of the people who have done the DIY route. I know I'm planning to try and document some of the process when I fit mine in the next 3-4 weeks!