.. if you skip chdk capable then any dslr will be even better (no need for magic lantern persé).
Canons CMOS cameras are better for long exposures (less amp glow, higher sensitivity, lower power consumption and thus less heat). Big pixels reduce the chance of hot/defective pixels and have lower noise (photon noise). My guess for long exposures is that the sd4000 is the best low cost camera, followed by the ixus 115/117. If you are spending more then take a 1100D or 2nd hand 1000D.
ps: better not look at the sd4500, its complex and open light path is prone to get full of dust.
