I see you've made a decision on the A100.
For low light and assuming close up work like the 28-70 you were looking at for the A100.
Pentax K-x with a Tamron 28-75 f2.8.
OR
Nikon D90 with a Tamron 28-75 or Sigma 24-70 f2.8.
Maybe a D90 with a 50mm f1.8 ( cheap, especially used ) and a 35 f1.8 ( also cheap ), The Nikon 50 f1.4 or 85mm f1.8 if you're feeling more flush with cash.
The Pentax can use old manual lenses and there area lot of relatively cheap 50mm f1.x out there. Of course you need to be happy with manual focus and on some lenses manual exposure.
The D90 should also be able to use older manual lenses.
And f1.8 or f1.4 is a very useful option in low light. Makes precise focus harder but in same cases indispensable.
--
StephenG
Pentax K100D
Fuji S3 Pro
Fuji S9600