If you want to retain TTL control then you're talking big bucks; Pocketwizards I believe are the kiddies at the moment but don't come cheap. Don't know if Metz has a lighting system like Nikon's where the flashes can control each other - this would be the first thing to check out, but again buying a series of flashes that interact digitally will be expensive.
The other way that most people go is to get optical or infrared wireless triggers that can be connected to each flash unit and triggered by either a master flash on the came or by a wireless trigger placed on the camera's hotshoe.
Flashinthepan sells these and they're good by all accounts - click
here to have a butchers. These cheaper trigger are usually limited in their range though so you can't spread flashes far and wide. A car shot shouldn't be too bad because you'll be working close(ish) to the subject but I don't know if these are dependant on line-of-sight to trigger each other. Ask FITP
The other slightly more expensive way is to get some wireless triggers like
Elinchrom Skyports (I use these). These are radio triggers that can work without line-of-sight and have a range of several hundred metres. They're £120 for a trigger/receiver kit and £70 for additional receivers. They don't have a hotshoe connector though so you have to either buy flashes with PC ports (like older Nikon Speedlights - SB24/25/26/28 and so on) or you get PC adaptors that fit on the hotshoe fo the flash. The main thing about this style of flash triggering is that it's all manual; the flash output is set manually, which is why the SB range is so popular. Vivitars are cool too.
Not entirely sure if you're Metz has a manual mode - the Metz website hints that it does - and if so, you're cooking on gas; this can be used off-camera as one of your flashes. Personally I'd get some of the cheaper triggers that FITP is selling and then get a couple of cheaper flashguns like
these that have manual control. A full set-up is going to cost you £100 or so and that'll give you enough lighting options to really get creative.
Think that basically covers it. Go to
strobist: lighting 101 to lean more about off-camera flash - it's a good site
