The Carriage for Two is limited to one per account and you must have at least the Elyrian package to buy one.
In order to create a new character or take over an existing character, you must use a Spark of Life, there's no other way.
Souls have no lifetime, they last forever unless they're destroyed by some unlikely incident. However, a character has a set lifetime of a year with no deaths. Even with regular coup de gras, characters will still generally live for at least 3 real life months. So, you're not risking much. If you compare it to a regular MMORPG's subscription model, you get far more out of one Spark of Life.