the factory twin dump manifold isn't that bad. really it could do with some porting, and larger secondary pipes made up. stopping the gaskets blowing out where the secondary pipes join is the biggest hassle with them.
a single muffer works fine so long as it is a reverse flow type.
a ke10 exhaust is not interchangeable, different floorpan.
as has been said, 2" max for an NA engine. don't waste your money on a stainless system (probably cost more than the car did). nothing wrong with a mild steel system, so long as it is thickwall tubing.