I got a pair of Audioque SD2.5 12's. They are basically brand new and havent even seen over 5 minutes of play time each. I'll sell them for $270 shipped. I went a different direction with my setup so I never used these. Let me know if your interested.
Honestly, it is impossible to tell how loud you will be. There are too many variables to be determined. But it will be LOUD to your ears. Someone on here has a 15 sd2.5 and they say it is nice and loud. So these subs in the right box on the correct power will be loud, yes. You would need an amp that does 500-600 watts rms at 1 ohm.
If I had the money.....I think I would go with a Rockford Fosgate T10001BD on ebay. It is rated at 1000 rms at 1 ohm but really does about 1400 rms. That is just under $400 shipped on ebay. If you really dont want to spend alot of money on an amp, you can look into the hifonics bxi1206D.