The "advantage" of a polymer rail system is only for the manufacturer -- make and sell them cheaply, and grab a piece of the low end market share.
The only polymer rail systems I'm aware of are the Fobus/First Samco/whatever handguard replacements, and they're not up to snuff for a fighting rifle. They might suffice for holding a lightweight flashlight but if that's the only goal there are other ways to accomplish that. I'd rather bolt a small piece of rail to standard handguards than use the plastic rail system.
Being on a budget is understandable. The drop in 2-piece handguards from Midwest Industries, Troy, and Surefire are all good options. The YHM version is servicable as well, although not quite as nice, in my experience. Getting a lightly used set of any of those is a great way to get an even better deal on a tight budget.
Zombies seek out and eat brains. Don't worry; you'll be safe if they attack.