I think the Ship will be a more sport version of the Going Merry:
a bit longer with the original sheep's head in front (they will salvage it).
As for the weaponry:
I think 1 (special model: faster reload) is enough. In Ussop's hands 1 cannon is more dangerous then 5 cannons on any other ship
Even if they offer to build Pluton I think Luffy will say "no"
I'm sorry for my bat english