How much do you intend to use it?
If it's fitted, and you're not paying a premium, you've not got much to lose. There are some things to look out for though:
- Is it a single point installation? The owner will usually be able to tell you but if not the giveaway is a big alloy ring on the intake manifold. Single point systems, especially cheap ones, cut a fair bit of performance off and aren't that great in economy terms, although obviously it's a bit cheaper to refuel.
- Multipoint systems have an individual injector per cylinder and are consequently much more expensive - but better on gas and more powerful.
-The certificate really isn't worth much - is it current for example? - so look carefully at how well the conversion has been done. Are the tanks in the boot, for example, because the XJ's boot is small anyway and putting a fuel tank in it compromises it massively, and also means you can't lower the seats to get more room (well, you can, but obviously you can't put long loads through the tank). Has the fuel filler nozzle been well located?
- Poorly set up LPG installations can kill any engine without too much effort. Does it run well on both gas and petrol? Give it a good hard stab on both. The 4.0 in the Jeep is a really lively, quite aggressive, engine and should pull it along well, without any missing or stumbling. Check the oil and coolant for any signs of cross-contamination.
- Does it switchover automatically from petrol to gas? This depends on the installation but make sure it's a smooth changeover.
- Does the heater work properly and well?
- How big is the gas tank? You can't fit as much in a conventional gas tank so if it's only got a small one then you'll have a very limited range - which is bloody annoying as LPG stations can be difficult to find (either just in general or ones that actually have any LPG to find) - meaning you end up running it on petrol anyway. And then find that it's got a half-size petrol tank so that doesn't go far either.......
- If the tank's in the back then make sure the rear springs aren't shot. XJ springs sag over time and fitting an LPG tank can exacerbate the issue. Look to make sure that the Jeep sits level when it's parked on a flat. If it doesn't (excessively so), budget about £200 minus fitting for a set of new leafs and bolts at some point
LPG can be a massively false economy on these things so tread carefully
Make sure it drives well too. A decent XJ should feel pretty taught, with minimal bodyroll for its size, and fairly precise steering. If it wallows like a boat and steers vaguely then it's getting on and not been overly well looked after
Instead, he's now looking at spending £3k on a rusting discovery. 
