I arrived at the dealership to find a 2006 STi with a salvage title, a CEL light on, and a non-working (locked) aftermarket double din system.
Walk away. End of story. No car is worth that, STI or not.
Other than saying, "Hi, I would like this car, but don't want to pay more than invoice, I know this means less profit for you, but hey, it's a sale", how do you get there?
You need to prioritize what is more important to you: the way it drives, or your price. Doesn't matter, just pick one.
If it's the drive, it's not fair to the salesperson to start discussing price if you have no intention of buying that day. In my opinion, walk in to the showroom (don't just walk around the lot - this is the #1 way to get a newbie), walk past the receptionist (helps to come in through the service entrance) and try to find the guy at his desk that looks like the #1 salesperson. Should be pretty easy. Ask him if he has a second, shake his hand, offer your name/phone/email/address/license and say today is not a price day, it's a driving day. Allow them to follow up with you - answer their phone calls and be honest. They only make money if you buy, not to test drive. Make your intentions clear from the get go.
(BTW most dealers require SP to go on test drives now, so I don't think you'll be able to fly solo. Getting the "Top Dog" gets you the best bets, since he usually runs by his own rules, but it again depends on the dealer. If he is running his mouth too much just fart or something, turn up the radio, something to make it clear you don't want to chit-chat. Just don't be rude.)
Do your driving. THEN start working with price. Again, Edmunds TMV is severely underutilized and will let you know what the market value of that car is.
On the other hand, if price is more important than the drive, do the internet quote thing and take the lowest quote to the brand that you live closest to. Drive the car to make sure you like it and then offer your price on the car. They either take it or they don't. If the difference is too big, go to another dealer.
Don't waste too much of anyone's time test driving. You're looking at econoboxes not 65k sports cars. None of them are fun to drive