To be fair, £2k is what I'd expect to be charged to refit a system on LPG these days, for a customer not lifting a spanner. There are a few things for why:
-Kit prices. Most come from Europe ; they are based in Italy. Given the customs issues we have at work, with a team around the clock to deal with any red tape that comes up, I dread to think how smaller retailers deal with it.
-Rates : With electric trebling (if not quadrupling for newer outfits or folks who have moved business), the owners have a choice. Work for nothing or charge accordingly.
-Other bits : Tanks have expiry dates on them. Something like that will be iffy. Older conversions will have copper pipes, which work harden over time; most newer conversions use Nylon hose. Then we are into whether the coolant hoses were cut well, or if they need to be changed. Some cheaper kits use questionable multivalves on the tank, which are meant to be there not just for filling, but for safety as well ; I'm sure we all know of folks who refused to have LPG kits
-Folks don't like touching others work. I've been there and done it, and frankly, it's alot easier and faster to start afresh than with someone's mess, where you have to think as the installer, or twit who installed the kit originally.
I used to installed LPG kits for a living, and 13 years ago, we were charging around £1.2k-1.5k for sequential setups. That was little less than the going rate. Sure, some folks charged less (sub £1k), but you could tell. They didn't work right for long, corners were cut, it took 30 minutes to fill the LPG tanks and so on and so on. Ours didn't put a foot wrong, with a minimal power difference between petrol and LPG. Time really does cost now unfortunately.
That said, almost every mechanic I know hates working on LPG setups, and working in manufacturing, you can see why. An Autodata or Workshop manual basically goes out of the window for anything more than simple tasks. Disturbing things may stop the engine starting for example (i.e LPG systems tend to work with an ignition source), so the cost benefits won't be as much as you think. I know mechanics now who wince over the odd LPG conversion they get in.
Yes, folks chuck mixers onto cars, which then castrate the performance on both petrol and LPG, along with having a stench of LPG (which with an honest tester, gives you no chance of passing the emissions), along with backfires and so on. To me, either do it right, or don't do it at all.
I've had cars with it done one from one end (XJ40 running on a Voltran mixer setup), to in between (badly installed Romano systems on an E39 520i 2.2i where it did 18MPG on LPG (No, I'm not joking!), and a LandiRenzo kit on a Mondeo we bought which we chased to work right, because we didn't want to install a new kit on it, to stuff which shouldn't work, but did work well with some thought (Zavolis on Mondeos ; Injector and vaporiser sizing accounting for a car working well, and doing 10k a month on the cars ; With larger piping into the appropriate multivalve, filling was around a 5 minute task as well).
Sorry if it sounds like I'm being big headed (I suppose I am), but I'd hate for others to deal with curse word others will put up with, due to cost.
How far are you from the Midlands, or even Lincolnshire? I'm happy to have a glance over if required, and maybe give tips on getting yours going.
Regarding the diesel conversion, there have been some good points here. Another one to consider are ratios. Sure the 6 pot petrols aren't revvers, but it will still rev higher and have different torque curves to most diesels out there, where most don't rev beyond 5k, and have nothing to give over 4.5k.
The BMW M57 with a manual 'box would be the easiest way. I think autos have been done before, but to me, I'd be tempted to go 2WD if you went that way. But, if an X5 isn't too nasty for you, I'd be considering just getting one of those instead ; they are pretty capable, albeit old now. I'm trying to think if there are other auto 4x4s dervs that could fit the bill. With emissions zones coming it, it may be a better time to buy a diesel than you think.
The drivetrain from one could be used in yours, but custom props would soon put your cost up quite a bit.
As said, it would be alot easier and potentially cheaper to get a car that works out of the box, for something that you need to depend on. The XJ suggestion sounds like a good shout if you want to stick with the Jeep family.
Merc MLs aren't too bad, but getting a rot free one can be fun. I was impressed with
RetroWarwicK's ML500, for what that did. The 5 pot diesels are agricultural, but they do the job well, being a Sprinter engine. That engine in a CLK 270 CDI I had, made it surprisingly sprightly.
Anyway, here are some cool shots of ZJs