Nope, I cant suggest another cannister. You dont HAVE to get an apexi brand though. I dont think theres anything to that style, in means of sound damening, so a cheap off-brand would be fine (as long as you use a resonator)
I like the ovals because its not as showy.
No, you dont need a high-flow cat.
A resonator is just what it sounds like. It removes the raspy, tinny, 'bees-in-a-can' sound that is associated w/ most...hondas. It will make the exhaust quieter, by lowering the frequency of the sound...a deeper tone. The longer it is, the more sound wave it can disrupt/absorb so the deeper the tone will be (therefore making it sound quieter)
18? Nah...
2.5dia 22" mild steel:
http://www.kteller.com/store/product...roducts_id/228
2.25 even has a 26" long one...but thats too much probably.
If youre concerned about the mild steel rusting, just spray a few coats of bbq paint on it, to keep the air/water off it.
EDIT***
Wait, yes I can suggest a different cannister. A short Magnaflow resonator/round can.