With a lager, to really do it properly, you have to have good temperature control; it's possible to simply keep it in a basement or somewhere suitably cold, but you'll never get the best results unless you can regulate the temp. such that you can lower or raise it in steps. A typical lager regimen is to start at 65°F, drop the temperature slowly down to 50°F over three days (usually with a 3°F step every four hours), then hold for another 7 days before doing a diacetyl rest (raising it in the same sort of steps to 60°F) for 3-5days. After that, it would be cold-crashed to 38°F and lagered for a month or more, depending on the style and the gravity of the beer (heavier beers take longer lagering times). All this is normally done before bottling or kegging.
Obviously, doing this takes a fermentation chamber with a good thermostat. That having been said, few kits are actually going to have lager yeast, even for a lager style beer, and those which do will use a temperature-tolerant strain that will behave more like a very neutral ale yeast at ale fermenting temperatures (similar to Cry Havoc). So long as you are able to keep the temperatures low (45°F-60°F), it should be OK.
BTW, where did you get the 6% ABV figure? Boston Lager comes in at 4.9% ABV, which is typical of the style (Vienna lager) in general. 6% would be more typical of the Vienna's big brother, the Oktoberfest. In either case, making a Vienna or Festbier (or Marzen, which is in the same group) with extract is tricky at best, because to do it properly you need a base of Vienna malt, a special toasted malt that AFAIK does not have any extract versions. You could use a base of light ME, and do a partial mash using Vienna malt, but the style really needs the Vienna to be the base malt to do it best.
As for kits, they can be good or bad, depending on the people who make them and how long they've sat on the shelf. I would recommend getting away from kits fairly quickly, and instead follow some of the established recipes on sites like HBT or the BeerSmith cloud.
Dan the Man Marzen sounds like it would fit the bill.