Hi,
I would like to offer you my expertise in the field of analog and digital electronics, pcb hardware design, embedded firmware and software development.
As you proposed there are two ways of approaching this project. If you decide with Rpi, this would be quickest and cheapest development. But the hardware cost per unit is bigger, it is not low power solution, it is hard to mass produce.
On the other hand if I design the whole system it can be power efficient, low cost per unit, small size and possible to mass produce. However development cost would be higher (I estimate 8 - 12 workdays, which would sum up to something about 1300 eur).
Development cost I offered you is for RPi solution.
These questions can help you decide which solution is for you.
1.) Is Minimal development time and cost important (yes -> Rpi)
2.) Is unit operated from battery, is low power important (yes-> own solution)
3.) Should it be ready for mass production (yes ->own solution)
4.) Small size is important (yes -> own solution)
5.) Quick boot time and robust design (yes ->own solution)
6.) Minimal cost per unit (yes ->own solution)
7.) Easy to maintain and upgrade (yes -> Rpi)
I can offer you both solution, depending on your preferences, If you could specify more about project - which gas sensors and would wireless be WiFi or Bluetooth low energy, then I could create milestones.
Thanks,
Vladimir