Welcome mate
there are different grades though they did try to simplify a little later in the years and just have a simplified fsport with several enhancement pack options that add things like sunroof etc in. I remember even myself being confused years ago.
Personally the Fsport is probably the better value for your buck in terms of what you get; which comes with nearly all standard lux trim options and some factory sporty tuning to give a sharper tuned vehicle on the road. On paper there are differences but in the real world the different 350 grades will feel 95% the same in car and on the road. Nearly all are equally comfortable in the Lexus way that you would expect. Fsport tends to have tweaked and more sporty look and feel like grill, panels, wheels etc. while I like it I have found, in the past, it can be polarising when looking at fsport models as som people are a fan vs. some prefer the look of non-fsport grades, particularly on other Lexus models like SUV etc. so yeah I guess personal taste plays it’s part here.
In the past I would have said I would be looking for an IS350 F sport model ideally post 2010 onwards as these tend to come with updated factory LED DRL lights and newer (at the time) tail lights. Which still holds true if you want a pure Oem car/look. However nowadays there are a host of aftermarket exterior/performance parts and more modern lighting options to help update your car (headlights, taillights), so if you find a mint is350 sport with older lights etc you could always go aftermarket to pretty much update any IS model you purchase to look/feel relatively newer...
in terms of differences, an example is, the earlier Lexus iterations and grades of car (sport vs sport luxury etc) Lexus would package them fairly closely with only very minor differences , I.e. e.g. fsport would perhaps only come with heated front seats, the sport luxury model would be the highest grade and therefore have heated and cooled front seats etc...). Different factory wheels etc.
Happy to be corrected on this one, but from memory the X model is often a limited run out model they typically do prior to an update/end of a generation of car, where in theory you’ll get a few options‘(including cosmetic) bundled together From factory for a decent value point, though In reality it’s a basically normal IS with a unique badging kit with minor options.
hopefully others will chime in here on what to look out for.
looking forward to you joining the owners club and sharing what you purchase, Hope the above helps.
cheers