A rusted control arm, even both rusted control arms, is no reason to junk a 240 if that's the only problem.

I put in brand new control arms (they weren't rusted) simply because it made installing poly bushings in the forward pivot easier than pressing out the old ones -- it's no big deal to change the control arms, really! And, by the way, brand new arms from Volvo were:
Left: p/n 1205680-0, $105.82
Right: p/n 1205686-7, $105.82
(these prices are with a VCOA discount)
[and you have to add the price of a pair of new front bushings]
which is pretty cheap considering the price of another used car of dubious history (at least you know your own car)!