FIR-Decimation and Low-pass filter (taps vs number of input points vs number of decimation stages)
I'll answer your questions first:


You should look at the final decimation factor when designing the earlier filters. Hence, for the first stage you can have a cutoff frequency of 1Hz and stopband frequency of 100000/10 - 1 = 9999 Hz, since this is what is folded/aliased into your band of interest. You would actually prefer to have several stopbands 9999-10001, 19999-20001, 29999-30001, 39999-40001, and 49999-50000 Hz /you'll have to normalize yourself...). This would require a rather low order filters, which is nice since it runs at the highest sample rate. For the next stage, 999-1001, 1999-2001, and so on. Higher order (about ten times), but ten times lower sample rate. And so on.
Roughly more stages => fewer multiplications per input sample (given that you do a proper design as above). This holds up to some limit which can be quite straightforwardly determined by searching some combinations. 
After the first stage you will have $1000 + 2 \times 127$ samples (with 128 taps) and so on.


If you want to get the DC value you can just average the sequence. This is, apart from the possibly obvious observation that the DC level should be the average "offset" of the otherwise AC signal, also what the DC component of the DFT evaluates to.