You'll find most people here recommend a wet test but the vast majority of buyers do not do this. Sometimes it can be difficult to wet test the exact model that you are interested in because most dealers only have the ability to fill a couple spas out of dozens.
Ozone is not necessary but a lot of places price it into their final sales price because everyone thinks they need it (or something similar). Circ pumps are also unnecessary, though you can make a compelling argument for one. I wouldn't let it be an ultimate deciding factor. Fyi, a saltwater hot tub simply makes chlorine out of salt which is sodium chloride.
My best advice is to stick with the reputable brands and pay attention to the dealer and how they do business. My short list would be similar to most here and includes; Hot Spring, D1, Jacuzzi/Sundance, Bullfrog, Artesian, Marquis, maybe Arctic or a couple of others. I recommend staying away from Cal Spas, Master Spas, the myriad brands out of that factory in tenessee that owns catalina and many would agree with that assessment.
Compare warranties and not just the overall year description of the warranty because some companies put weird exclusions or prorations in there. Look at fit and finish. See if you can get them to pop off a panel and look inside. At least sit in it dry if you can't wet test. Look at the types of jets used. Smaller jets tend to be cheaper, high pressure/low flow jets which don't feel as good. to a lot of people.
That's probably my best "where to start" advice.
