I agree it's important for you to test ride SOME bikes, even if they're not the bikes you think you want. For one thing, after you actually ride a few, it will be easier for you to narrow down what you're looking for.
Also agree about making a prioritized list. There's not one bike that I know of that's ideal for ALL surfaces/types of riding.
Be brutally honest with yourself about the kind of riding you will actually be doing MOST, and get the bike that will work for that. You may have dreams about mountain biking (for example), but, with your medical issues and the length of time it's been since you've ridden a bike, how reasonable would that be for you? Get rid of the "maybe someday I'd like to ride...", or "it might be nice to be able to ride..." thinking, and stick to what your majority riding is going to be like - that will help to narrow down the choices so at least the bikes you're considering are all in the same category, with similar features.
Finally, not sure about the wisdom of you picking a bike for your girlfriend - that sounds like a recipe for disaster
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