When evaluating CBCT, I recommend downloading BlueSky Bio software because it is free and very feature rich. If guided implant surgery is one of your goals, you can use BlueSky Bio to design surgical guides . The BlueSky software has a great video tutorial library to guide you through basic functionality to the more advanced features, so, it is easy to learn as your needs evolve.
The next step is to request DICOM volumes from various manufacturers - both small and large volumes. Ask for volumes with restorations so you can see how the unit being evaluated performs when restorations are present. All CBCT units will produce artifacts such as scatter and beam hardening in the scan when restorations are present. This is why it is often necessary to use a scan appliance (basically an impression tray with radiopaque markers) and a dual scan technique to allow for the alignment of a scanned model and the patient's CBCT. When those two volumes are combined, a surgical guide can be planned and 3D printed.
Open each different CBCT volume in BlueSky and review the volumes in each view - axial, sagittal, and coronal. Take screen captures from the volumes to compare image quality and scan accuracy. You will find that not all CBCT units are created equal. You will also have a better idea of how the image quality varies and what you may be giving up in trying to save money. Make sure that you ask the manufacturer for volumes taken in different modes as well, and, insist that all volumes provided are ones that were taken on live patients and not a dry skull.
To give an idea of the differences among CBCT units, I have taken axial view screenshots from five different machines. It is important to note that the exact same slices are not shown between images, and the last two views are from different patients. The point of the images is to illustrate the difference in quality and detail among different machines.
The best evaluation would be a scan of the same patient on every CBCT unit that you are interested in purchasing. You would also need to have the scan done in various modes to evaluate the quality of a high-resolution mode versus a "low dose" scan. Unfortunately, a scan on each machine of interest is not typically feasible. Therefore, requesting volumes to review in a third party (free) software is a good way to start comparing volumes. In the process, you will also learn about some of the limitations of CBCT. For example, you will not diagnose decay when restorations are present due to dark artifacts-- referred to as beam hardening-- that can easily be mistaken for decay.
The following image shows beam hardening. Look at the dark area present under the restoration of #14 and the distal of # 13. Beam hardening and the streaking referred to as "scatter" are two examples of artifacts that are common in CBCT volumes. Manufacturers will vary kV, beam angle, rotation, and reconstruction algorithms to reduce beam hardening and scatter. A google search for "artifacts in CBCT" will return many scholarly articles that explain different types of artifact in CBCT volumes.
I also recommend that you watch a few online courses that will help you familiarize yourself with what you are seeing in CBCT volumes. DentalTown has a couple of excellent online courses that will educate you about what to expect when you start reading CBCT volumes. Dale Miles, who is an internationally recognized authority on CBCT, has a course on DentalTown. His website www.learndigital.net is a great resource for anyone investigating CBCT, or who has CBCT and wants to learn more.
It is also good to have access to a radiologist who can read scans when you are not sure about what you are seeing in a volume. One site that we recommend to our clients is www.oralradiologists.com because it is easy to upload your volume to their site and they do a great job. Our clients have reported that they are very helpful if there is an issue with uploading or if they have questions regarding the report.
Hopefully, this will give you a few insights that may be helpful in your CBCT search. As a disclaimer, I have no financial interest in BlueSky Bio, Dale Miles, or Oral Radiologists.