[Morgeruat]

as for how to make one, without building a custom class take a standard fighter, specialize in katana (need weapon focus, Katana first) and then get feats that seem appropriate, such as cleave (low level version of whirlwind attack) knockdown might also be appropriate. you can also take a few levels as a monk, to get the AC bonuses and some of their other special abilities.

as for magical katanas there a few in chapter one but they are farily rare, you can buy one from a merchant in the docks district, and have it upgraded to the sword saint katana (Kensai are often called sword saints)