Brewing a kit lager is pretty much the same as an ale kit. 'Proper' lager yeast ferments much cooler than ale yeast, and then the beer requires cold storage (laagering). All difficult without the ability to do a temp controlled ferment.
Kit lagers will use a neutral ale yeast to produce something that is lager like, even if it isn't really a lager. The Coopers kits mentioned by Tippler are rated as good by a lot of people. Have a browse through the kit reviews section.
One problem you might have is that you'll struggle to get lager levels of carbonation in a plastic keg. If you're happy with lager that's less fizzy than 'normal' lager then no problem.
