1.8 and 2.0vvt variants are probably your best bets. The 1.6 is underpowered and has a few niggly problems, while the 2.0
can be very problematic even at low mileages and is probably best avoided.
I've heard no feedback on the reliability of the 2.0
models, but based on Renault's track record with turbos, I'd be inclined to steer clear unless you really want the extra welly. The
might complicate the LPG conversion too, though I have no real knowledge of the subject.
3.0 v6 is generally a very reliable donk, though like most V6 FWD cars, it's shoehorned under the bonnet so tight that it's a nightmare to work on, and servicing costs go up accordingly. If you're looking to convert to LPG, then the fuel costs of the V6 won't be as significant, but if your motive is to spend as little on fuel as possible, you'd still be better off with one of the smaller engines.
If you're thinking of jumping ship, then take a few other models out for a test drive before you commit. The Laguna's good for a smooth ride, but reputedly doesn't handle as well as the likes of the current Mondeos. If you want something that'll eat up the motorway miles without giving you backache, then you won't go far wrong with the Laguna (back trouble is one of the reasons I've stuck with Renault - it's been very rare that I've had backache in any of my Lagunas, and I used to get it all the time in previous cars.) You'll also be disappointed with the number of 'toys' that most other manufacturers offer, having been spoiled by the spec of the Renault.