To be absolutely correct, that car is a Biturbo Si rather than a Biturbo S, there are a few minor differences most significant being the introduction of fuel injection for the Si. The Si also had the intercoolers moved to the front of the car rather than either side of the intake plenum as on the S, the NACA ducts on the bonnet for the Si are therefore largely redundant.
Interesting that this car is on a K-plate despite being a 1987 car but not the first time I've seen an imported early car on later plates.
I think this particular car looks really nice and is probably priced about right based on its mileage and the described condition. The Biturbo S and Si are very collectible in Italy and are starting to command fairly high prices, particularly the limited edition "Black" version. What may possibly count against this particular car is that it is not in original spec, a standard car would have a grey bottom half, the smaller early type bumpers and would only have twin exhaust outlets on the right side rather than the 4 pipe setup on this car. I think it looks great as it is but it might upset the purists.
This is how it would normally look as standard, note different front and rear bumpers to the car advertised plus grey lower body:
A picture from the Biturbo Si brochure showing the colour schemes, red, black or silver with a grey lower half:
Yours anorakingly
DickyB