I've purchased both of my cars new through Tesla, including transferring my SC01 FUSC over to my new 2023 MS LR in end of 2023. If the car went from the first owner sold directly to the local 3rd party dealer, then there is a good chance you are OK, but no way I can guarantee that. Does the current Chrysler dealer have access to the car via the Tesla app, meaning the ownership through Tesla was transferred to them? If so and it still has SC01, then that gives me more confidence you'll be OK, but it's so hard to be 100% certain on SC01 transfer unless your are dealing directly with the prior owner that I always hesitate stating conclusively that you will be OK.
My suggestion is if this car is one you'd buy at the current price even if it didn't have FUSC, then go for it. If you still get FUSC, great, that's just a bonus.
Actual savings from FUSC is so variable based upon you specific use case, but if you want to do some work, you can figure out what FUSC has saved you on your current car. Download all your charging history off the Tesla app. If you do that as .csv files, you can then pull those into something like excel. It will show you what the unit cost ($/min or $/kWh) for each station along with the number of minutes/kWh. While it will show zero cost, you can use that rate data to calculate what the cost would have been if you had paid. That's the most objective way to see what FUSC actually has saved you.
Good luck on your decision.