In-house micro takes a lot of labor. Make sure you include labor in your ROI calculations. Often people do the math on the media/tools and go "$5 a test!!", and then forget that you need to conduct the tests themselves, manage all documentation for your tests, equipment, and inventory; order and manage inventory, back-up your testing databases, more calibration and verification, accreditiation fees and time, autoclave waste stream, etc.
Then there's the liability involved. IMEX in-house testing leads to more false positives with increased time to verify since the samples aren't already at a lab that can do confirmatory testing, and sneaky false negatives unless you are going for an ISO accreditation both in certification and in spirit.
The ISO standard likely has some credential information. But basically you need personnel with the experience/education combination to determine proper testing methodologies and interpret the results. Rapid testing tech is making this easier and easier for basic pathogen detection in most foods and environmental surfaces, and they will often provide training and support to make sure you can conduct the testing competently and in-line with an official standard method.
Depending on what you're testing, if it's just qualitative indicator organisms for environmental surfaces etc., you could design a validation scheme where you send duplicate samples out to accredited labs at some intervals to make sure your results are reasonable. If you're doing any sort of pathogen testing, I would recommend at minimum doing proficiency testing like AOAC offers and analyzing your performance when the results come in to validate your in-house lab.
Some good questions to start with:
1. What are the goals of bringing this in-house? Savings, test turn-around time? Could this be done by switching to another outside lab?
2. What tests do you want to do? Indicator organisms for sanitation verification and environmental monitoring? Or Pathogens on Products and FC surfaces?
3. For the tests you want to do, what's the proper equipment, media, and waste stream. Will any of it require refrigeration? How many incubators, homogenizers, water baths, etc.
4. How will you manage the records of incubation times, media lots, etc. for each individual test? With this require database software?
5. If the lab isn't staffed 7 days a week, what days will you not be able to initiate tests? Does that impact your decision based on turnaround time for products produced near the weekend?