[Perfectlap]

well I think you're missing some options that end up making the car look like a 2.5. Namely 18" wheels, Litronics and aero. But to be fair I don't believe that these were available options in '99 (which is why it looks like a 2.5 at first glance). So basically the bulk of the value in your car boils down to the going rate for a low'ish mileage 2.5 engine. Porsches in general are worth shockingly little without their engines.
I'd say between $9-$10K is fair.
Any higher at all and you can probably land a 3.2 also on the smaller 17" wheels, also minus litronics or aero treatment with slightly bit more mileage but as long as its in the 30K - 50K range its still the more sought after option because you're up to 250 HP. That's the problem with Porsches, even if you have a low miler to sell, there's always a similar Porsche with more power for sale right next to yours and the market doesn't seem to give somewhat bigger engines that much of a pricing bounce thereby making those the first to sell.