ADC sampling frequency - Noise filtering
You can sample them 2 to 20 times per second and then calculate a moving average over a second.  your response time will be about a second and there should be no noise if your electronics are even remotely reasonable.

Unless you have some interesting micro-turbine and want to measure accurate fuel consumption you are not likely to need very fast response from your filters or ADC if your sensors only respond at 1 to 10 Hz.

EDIT:
If you have or think you will have non random electrical noise much greater than 5 times the resolution that you actually need (not what can be calculated) you may gain benefit by adding analogue filtering.  The cut-off frequency should be around half of your sampling frequency.

However any low pass filtering above your desired response frequency would be a benefit though the digital averaging filter is a low pass filter already that can eliminate a lot of random noise sources.  if you have noise near some multiple of the sampling frequency then the aliased noise may cause a beat signal in the output that is not actually there, an analogue front end filter would minimise this risk.