Fun luxury coupe sporty car for Lexus Is250C replacement

How about an Audi A5 or S5? Room for 4 in a pinch, very sharp design, inside and out, comfortable, quiet.

Even a cabriolet version if you want a convertible, and particularly if you buy CPO (certified pre-owned), you get a great car and a great price and with the peace of mind of a great warranty.

If, as you said, you're willing to consider used or CPO you really should consider the Volvo C70 T5. This model was discontinued in 2013 after a long model run. We had a long term rental back in 2012 and after a couple of weeks living with the C70 found it to be a most underappreciated vehicle.
It's not a race car and sports a mildly turbocharged 5 cylinder engine that Volvo has used for many years. Power is certainly adequate. Ride quality is excellent, seats are extremely comfortable, the car has a usable trunk, even with the top down AND you can actually put a couple of adults in the back seat for an evening out. The Platinum trim features an audio system designed by Bang & Olsen that needs to be heard to be appreciated. Our vehicle has a Nav system that is a little awkward by todays standards but we don't really have much use for it anyway so it was not important.
Another advantage to owning a T5 is that you won't look like everyone else on the road. Styling is elegant and practical. We looked at the Lexus is250C's and while a nice car it just didn't have the elegance and the practicality of the C70. We also looked at the BMW. I'm sad that Volvo discontinued it but we're pleased to be owners of the last of the line.