What is a suitable sub&amp for me?

SQ is out of the question with a loud exhaust.  The tone of the muffler wipes out any quality you hope to get with your sub music.  Use a quiet car for SQ.  What you need is extra power to overcome all the noises your car makes.  Running a sub at 4 ohms with a loud car is a waste of amplifier, IMO.  I run at 4 ohms, but my car is quiet.  Look for a minimum of 500 watts at 2 ohms into either one or two subs that will match or exceed that power.

You also need to upgrade the damping in the car, the sheet metal is rattling.  Amps are easy to find now for that power range, and a class D mono amp would suit your purpose.  If you listen to subs for comparison...have your car right there with the tailpipe screaming so you can hear the sound in a real-life situation!

Actually, the sheet metal vibrating from the road is probably causing most of the other noises you're hearing.  You can deaden those vibrations, the noises they are causing, and muffle the sound from the exhaust with deadening material.  

I also drive a very loud car and doing the doors and trunk helped quite a bit.  I plan to do more when I get the time.  Check out raamaudio.com, b-quiet.com brown bread, and dynamat xtreme bulk pack deals on ebay. 

In fact, now that I'm talking I'm going to go ahead and disagree with stevdart on this one.  Pros, as always, please correct me if I'm wrong on this.  Say our friend here wants to listen to his music at 90db with a small 300 watt 10" sub system.  He spends some time matching his gains, etc. and his mids, highs, and sub all play well matched around 90db.  Say that they are all playing well within their limits.  Add in the car with the loud exhaust.  The muffler kills some of the bass/midbass.  Short of any upgrades he has two options as I see it:   turn everything up with the volume control when it gets loud or keep the volume / gain up on the bass all the time to compensate for when the muffler is being loud.  If he does either of these two things and his sub amp / subwoofer are still well within their capable playing levels, what good will an extra 200 watts do?  Maybe I totally don't understand, but as long as his amp can push his sub as loud as he wants with room to spare, what is the point of throwing more watts in the trunk?  Why put the extra watts back there? 

Sorry, but I also have to disagree with stevdart. Sure the exhaust is going to interfere with the bass notes a bit, it really isn't that bad. I have a 97 Full size chev. with true dual exhaust with dual cats and turbo tube mufflers (just barely street legal) and I am only running 250 watts to my sub under the back seat, and it's plenty loud. I see that kfr01's system is Extremely similar to mine, so you know what I'm talkin'  about.  Just look for a very efficient sub in the correct size enclosure with a good quality amp, and you should be alright.

As far as the PG amp - I haven't looked it up, but it sounds like it has a regulated power suppy, meaning that it will produce the same power no matter what you hook to it (well, you know, within it's limits). There's more to it than that, but that's the basic gist of it. Some other amps with regulated supplies that come to mind are JL Audio slash series, some Memphis, .....