The frequency range will vary depending upon manufacturer, but generally 30 to 32KHz is recommended for mechanical inspections. For steam on other contact listening like electrical scan through a metal cabinet(20 to 30KHz) for open air electrical and leak detection (close to 40KHz).
I would purchase an ultrasound listener for the ability to use it for electical scans, leak detection, and mechanical applications (espeically since you are in a power generation plant). You can always get a lubrication fitting as optional equipment or you can make one. There are two to three good manufactures depending upon you applications.
Before base line readings are collected, it is recommended that lubircation be performed and given one to two weeks for the lubrication to take. Then baseline readings can be taken ... You are baselining the bearings at their "best" condition (albeit some wear will be evident on certain bearings). It is important to remember that each time you collect ultrasound readings, that the equipment is at the same load / rpm as the time before as this will affect your readings. Keep records as to operting condtions at the time of sampling and also instrument settings. You want to repeatable sampling program.
Ultrasound, if used properly, can filter very quickly and cheaply which equipment should be scheduled for vibration.