You do realize that you're going to bargain them down from the asking price, right? If you look at a car that is listed for 20k and you have 18k to spend, that's what you need to bargain them down to. Eventually you'll find somebody willing to work with you on price to get the car sold.

Also, you like the Sti, why not get a WRX? They should be in your price range and with some basic mods like a chip, uppipe/downpipe/turbo back, intercooler hoses, should be a fun, fairly reliable daily car with about 100 mph traps and in the 13s somewhere.