> I need something to support it so it won't sag, and something to put legs on.
I'm a big fan of "L girder" benchwork. It essentially forms a beam, and is nice and strong as well as easy to bolt legs too.
I've seens lots of layouts made of various lightweight materials etc, but if you want my suggestion, don't scrimp on your benchwork. It's literally the foundation of your layout. I always use 2 x 4's for the main framework, this isn't the place to save a few bucks on wood.
You want something you can lean on and it won't sag or flex. My standard is simple, I have to be able to stand (or kneel etc) on it without any sway, wobble or noticable sag.
Provide solid legs, and some bracing is helpful too. Ping-pong tables are fine for a youngster's oval, but if you're building a permanent layout, go for the solid foundation.