Please, those wheels do not give your car 4x4 status. And I don't think you track your car since you have limited funds to spend on it, so I wouldn't let the weight stop you. Personally I think the 18's look a lot better and are the right kind of mesh for a sedan, they give it a bit of a sport touring look, but still classy and not outrageous. Very tasteful, and better than a small 5-spoke imo.
The only concern I would have would be the tires on the 18's. If they are in pretty good shape then I say pick these up. But if you go look at them and the tires are bad, I would negotiate the price. Or decide whether or not you are willing to buy new tires.