It can be as simple or as complicated as the customer wants to make it. One of the problems with the live viewing of an NVR/IP system is the delay in the video. Some customers freak out about that, some do not. Also, audio/video sync on most IP systems is off. Again, some P.D have issues with this, some do not. Louroe is THE big name in external microphones for law enforcement. Their mics are fantastic and do have a lot of options for styles and mounts.
One of the bigger issues with interview rooms is the offloading of the video... meaning how it's offloaded and what file type it is offloaded in. You get into issues of watermarkes, proprietary video files vs non, frame counters, on screen date/time. Etc.
I've seen some police departments use stand alone Sony DVD recorders with an analog wide angle camera and Louroe mics as VCR replacements.
Some departments that already have IP systems will dedicate a camera and mic from it as a 'room' but I'd say the majority of them do stand alone systems for interview rooms. There are a handful of companies that custom make interview room recording systems, but they are extremely expensive. One of the other requests that comes up often is CMS... or case management. No traditional VMS has the CMS that they want, but some of the custom units I mentioned have been written from the ground up to be interview room systems.
So, to answer your question, there is no 'best way' to do it. It's all about how specific the customer wants to get. I personally think that a standalone, single channel analog system with a good Louroe mic is best, but I'm biased..... I helped design one. I've heard of customers using SD card based mobile DVRs for their interview rooms because they were very simple to use and offload the video from.
Ask the customer about their specific requirements, regarding to:
-Watermarks/date time stamp/frame counter/proprietary video format
-is anyone watching the video live? Is a video/audio delay ok?
-Do they have an existing system at their department that they can expand and use, or do they want it to be independent?
-is long term storage of the video required? IF so, how much?
-Is a CMS required?
-How 'technical' is their department? Will they be able to handle and operate an NVR system, or do you have to go analog or HD analog?
-How are they planning to 'activate' the system? A manual button is requested more often than you would think.
I was formerly a product manager for interview room systems and helped design several. These were all issues and questions that came up pretty often.