Best of luck with your project & perhaps I can be of some assistance here?.
I used to manage a central testing laboratory which analysed samples for microbiology/ chemistry/pathogen and was staffed by 24 technicians, testing for 16 production sites across the UK. The site where the laboratory was located was lucky because the laboratory was at the bottom of a hill (meaning the water course flowed AWAY from production) and it was also a transport hub (meaning chilled samples could be brought to the site daily, along with routine foodstuffs FOC). The laboratory was accredited to ISO17025 (there is a cost involved in this), but if it hadn’t been formally accredited, we would have been obliged to follow the principles of the standard to ensure accuracy of results (BRC mentions this & probably other standards as well?). Maybe you could get hold of a copy of the ISO17025 standard to see what is involved?.
Having the laboratory on site was only economical because we cross-charged the other sites for conducting their testing & were able to purchase the reagents/media in bulk (reducing cost). Every year I was obliged to provide a cost comparison of our laboratory outgoings, including instrument calibration costs, media, chemicals, salaries plus revenue from other sites for conducting their testing and compare these to external testing costs.
As I recall using our inhouse laboratory was cheaper by approximately £45K a year (across 16 sites) but this was primarily due to economies of scale when purchasing disposables/chemicals/media and deployment of staff. Had this been a stand alone facility I’m certain it would have been cheaper to test externally.
But, if you can make it work for you financially that would be ideal, as so much information about microbiology & pathogens seems to have been lost because we are increasingly disconnected from our 3rd party laboratory service suppliers!.