It's very hard to find good steering in a recent car. I like the EX, but its steering is fairly numb. The steering in the related G35 / G37 sedan is a little better. But still far closer to your IS than your 240SX.
The 2014 and up Mazda6 might have the best steering among mainstream midsize sedans, but it still feels like a midsize sedan, and so not nearly as sporty as the 240SX. The smaller Mazda3 feels more agile. Then again, it will probably be hard to find either as a 2014 within your price range.
Instead, I'd advise another coupe.
The Scion FR-S and Subaru BRZ (essentially the same car) can be a lot of fun. Owners have reported some problems with them, but nothing major, and most should have had these problems fixed by now (a chirping fuel pump, an uneven idle, tail light condensation). The main downside for these cars is that they're too noisy on the highway. Around town, no issue.
If you do want to trade some sportiness for lower noise levels, look for a 2013 or newer Hyundain Genesis Coupe.
I've checked, and it's possible to find 2013s of any of these for under $18,000.
I'd also suggest a Mazda Miata, but it has no rear seat.
I personally own a Mazda RX-8, and it steers better than anything but the Miata, and is far more livable on the highway than the FR-S/BRZ, but fuel economy is poor (high teens suburbs, low 20s highway) and reliability is iffy. Cheap to buy, though.