The unit we have is a Signstek ST-05B which we purchased on Amazon. Today, the price is about 25% less than what we paid back when the drive-in church model was first getting traction.
I run a line-out signal from the PA system to drive the transmitter, and I boost the treble a bit for optimum sound quality. In technical terms, this is required by the US standards for pre-emphasis, which is a legacy requirement from the early days of FM radio. In simplistic terms, higher frequencies are severely cut by FM receivers in the US, which is not the case in the rest of the world.
A friend in Washington state uses this same unit without a PA system and runs her keyboard into the line input, and the pastor’s microphone into the mic input and mixes the sound right on the transmitter itself.
For frequency selection, you need to find a clear, or mostly clear channel in your church’s location. I used a website called http://www.fmfool.com and looked for the lowest received signal strengths, and the largest gap between stronger stations, to reduce the potential of interfering with a neighbors radio. Thus the only interference left that one might run into is gyms or fitness clubs that may transmit TV audio or music on the FM bands for their members.
This Signstek unit does not have any way of observing the input level. If it’s too low, and it won’t sound very good in folks cars. If it’s too high, it will splatter across the FM band. What I do is set up a portable radio, and slowly increase the audio gain on the transmitter, and leave it at the minimal setting that produces good quality sound on par with local FM broadcasters. I then scan across the FM band with my car radio to ensure there is no splattering on other frequencies. Before each service, I check the performance by walking the parking lot with a portable FM radio, listening for distortion, and/or gaps in coverage. Such issues would be a function of antenna placement mostly, but it also is a good cross-check to make sure everything is working correctly.