Two words for you... timing belt.
Most Honda four-cylinder motors are interferance designs. So if/when that timing belt breaks you'll find yourself with a banged-up engine. Timing belts need to be replaced at a regular interval, usually every 60 or 75,000 miles. If the previous owner does not have a record of when this was done then I would look elsewhere, or at least talk the price down to include the cost of replacement.
As with all things Civic they are highly modable, but it isn't clear that you want to do that. But an Si is already pretty quick, 105hp and barely 2000lbs curb weight = fun. Unless you are planning to mod it or anything like that, a 15 year old car is one you buy and drive into the ground until you can afford something better. And if you are not able to drive it into the ground you sell it to some other kid looking for a fun car to drive into the ground.
One more thing, as a general rule cars are poor investments. Unless you find a mint condition Dusenberg SJ
for $500, then I would suggest picking that up.
Stop drop and roll will not save you in hell. --- seen on a church marque in rural Virginia