I'd vote for the HP-1xC line myself. You can get 5 different calculators (financial, 3 x scientific and programmer) for nearly the effort of the first one. The HP-16C (http://www.hpmuseum.org/hp16.htm) would be especially helpful as it can easily be converted to calculate with a 12 bit with carry width and octal operation with just a few keystrokes. Even does 1's complement/18 bits if you happen to have a PDP-1 lying around.