The standalone (which is not out yet) is going to use a modded engine similar to take on helicopters which in turn is a modded version of ARMA 2's engine, so using ARMA 2 as a base to build for is actually the right idea.
$400.... you could build something to run it but it wouldn't be pretty on that budget you may be best off looking on Ebay for a 2nd hand rig.