Milos, There is only one question that you need to ask at your company: "Are we planning to run metalised (foil) film for the bags?"
If the answer is "No" then the best point of inspection is in the finished bag. Contaminants cannot enter a sealed bag without leaving a mark (hole). Inspection of the finished sealed product is ALWAYS the No. 1 desired place for inspection.
If the answer is "Yes" then you need to consider two options:
1. Use an X-ray on the finished bag. X-rays can easily detect metal contaminants inside a foil bag, but metal detectors can really struggle with this if the film is thick.
2. Use a drop through (throat) style metal detector to detect metal in the product before it enters the bag. This is not ideal (because a contaminant can still enter the bag after the MD but the chance of this is very small) but commonly done on a VFFS machine.
Other comments:
Don't be talked into buying a "Ferrous in foil" metal detector unit - they are a waste of money - they only detect ferrous metal and most food processing equipment is S/S, i.e. NON ferrous metal.
The speed of the product dropping is irrelevant - whomever supplies the equipment just needs to calibrate the unit correctly.
A metal detector can achieve as good a result on dry product like cereal as an X-ray (sometimes even better) so long as you stick with the smallest possible opening (regardless of whether it is a conveyor or a throat unit).
Rejecting the bag off the line should be 100% accurate - but again it needs to be set up correctly by whomever supplies the machine.
Both units should be roughly the same cost (the throat unit will seem cheaper at first glance, but if you use the VFFS take away conveyor to mount the MD on, then the conveyor system should actually be cheaper i.e. you were going to buy a conveyor anyway for this job, so use the same conveyor for the MD).
Make a list of reputable brands (quick internet search). Reputable brands will all cost about the same and deliver similar results. Then I'd look at ease of use and service back up; these are often overlooked but are very important.