Is it an RSX Type-S or standard? If it is a standard RSX, then the Civic Si and the RSX actually have the same amount of horsepower (160 hp). They are mechanically similar, but the RSX looks better. 100k miles is not too many; I have seen these cars reach 200-250k mi before needing major repairs. If you buy it, make sure to change the oil and filters regularly to avoid engine trouble. Since it's at 100k, it will also probably need a new timing belt.