Usually, if I find a fairly common Marx 6" tin windup car - in good condition like yours - for $10 bucks at a train show, I feel really good about the purchase. Walt's guide values it at $17, so $8 is a can't-pass-it-up deal to me! You might find it cheaper on Ebay, but shipping costs would easily put you over that price - it would probably cost 8 bucks for shipping alone.
Too bad you couldn't get the other two cars your were interested in, but I understand the luggage space & cash limitations. BTW, I may try your foam idea... it could be disguised as coal or gravel, and it is worth a try to dampen the racket. A train of Marx tin running a windup speeds can make quite a roar!
