There's a commonly accepted belief that the Laguna's lights are multiplexed, though to date, nobody with sufficient technical knowledge has been able to absolutely confirm or deny it. Playing it safe, I bought the official Renault wiring kit for my 2002 L2, only to find that it basically did the same job as a bunch of Scotchloks (albeit with plug-in connectors so you didn't have to hack into the existing wiring.) There were no bypass relays in the kit, and the only 'gizmo' was the standard trailer flash buzzer relay.
I've had my 2005 L2 wired by a professional, who was also of the opinion that bypass relays were unnecessary. He has wired the car without them, and so far I've had no problems.
Of the manufacturers you've mentioned, I'd definitely go for Witter. This bunch come highly recommended by the guy who did mine - apparently they're a great quality budget bar http://www.tow-trust.co.uk/
If you want to play it safe with the wiring, then just buy a bypass relay. You'll need to source a suitable 12v permanent live feed in the boot area, though if you're wiring a 12S, that shouldn't be a problem.
Finally, if you have the hatch rather than the tourer, you'll need to remove the left hand interior boot trim to get at the multi-plug for the rear fog and reversing lights.