Unless you're specifically trying to build an "urban wasteland" or "urban warfare" type of scenario field, skip the cars. Even after all the work needed to safe it (removal of the glass, gas tank, any oil, weld the doors, etc.) they're a pain to move in and worse, almost impossible to move once placed, if and when you might want to reorganize the field.
You're kind of overthinking the barricades. Unless it's a well-run scenario game with missions and whatnot, the players don't care whether they're hiding behind a well-painted plywood Sherman tank mockup or a stack of old tires. The game is the interaction between players- bunkers are just something the players use to keep from getting shot. And for that, a pile of old cable spools works just as well as a $100 inflatable beer can.
One of the best local field we ever had was Rocky's out in Caribou Hills, and he used three kinds of very basic barricades:
Two oil drums welded together to make a pillar, stacks of tires (the round ones are big tractor tires wrapped with heavy plastic) and two kinds of constructed bunkers, both being simple wood skeletons with heavy plastic sheeting. The domed ones had "ribs" of PVC pipe, as I recall.
The plastic sheeting is a product similar to Typar, but it's solid plastic, not the woven stuff. It was very heavy and durable, and basically made a rigid copy of an inflatable bunker.
That's all you really need. And one thing I like about that setup above, is it doesn't look like a junkyard. There IS a PR aspect to the appearance of the overall field, both in your location right alongside the highway like that, and to a prospective customer- especially the mothers of younger kids. A field like that above looks somewhat friendlier (and safer) than a yard full of wrecked cars.
If it were me, I'd spend some time making about half a dozen each of those triangular and domed barricades- they're just simple wooden frames with the plastic stapled on- and pick up some oil barrels. (Just be careful when you go to weld 'em together, if they ever held anything remotely flammable.) And all three are lightweight and easy to move when it comes time to reorganize the field.