The size of the pipes will change the sound a bit, but that's mainly a function of the muffler(s) that you use. Without knowing too much about Arvin stuff I can't really say one way or the other what it might sound like. I know that they are the people who make the original Cherry Bomb muffler which is probably what the guy is talking about using as a resonator. This is a Cherry Bomb:



Used by itself as just a muffler it's loud as all hell, but used as a sort of pre-muffler or resonator for another muffler it helps reduce the overall noise level by a certain degree.

As far as the actual muffler at the end of everything I would definitely go with something that is full stainless. Lots of companies make stainless mufflers; Dynomax, Flowmaster, ES, etc all have assorted universal fit stuff.

You should find out how much this guy charges to do a full stainless system. If that's what you get for your $300 then that's one helluva deal, but if the price starts to go up then you can get a pre-made system for not too much more money.