A question on filters
Too many things to comment here:

1- Decibels and thousands never go together. -1000dB means $10^{100}$ power attenuation (or $10^{50}$ amplitude attenuation), which is A LOT (an astronomical lot!): the noise level of any electronics, the DAC quantization noise, or the noise introduced by digital arithmetic will make your 1000 dB attenuation useless. Furthermore, designing a filter for such attenuation will normally result in a order so high it is impossible to implement.

Consider maximum gains or attenuations in the 100s of dB. As a reference, signal to noise ratio of CD encoding is around 96 dB. Or a satellite radio link can have a free-space attenuation of hundreds of dB.

2- The $20 N$ dB/decade attenuation of (low-pass) filters is an asymptotic slope. The actual frequency response depends on the type of filter (Butterworth or Bessel have the smoothest response). Which to choose depends on your application.

3- How does your system work?

analog signal -&gt; ADC -&gt; digital LPF -&gt; downsampling -&gt; average

or

analog signal -&gt; analog LPF -&gt; ADC -&gt; average

Why don't you place a big RC filter and sample at a lower frequency? You'll still get your average.

In any case, you always need a low-pass filter before the sampler to avoid aliasing.

4- for your (c) question: if you consider amplitude response of the filter, then it is $20 \log$. If you consider magnitude squared (or power response) then $10 \log$.