I don't see the problem in using Car Audio subs instead of home theater audio subs. Especially since I can use the car subs in a car later when I get one.
Car audio subs are car audio subs for a reason, they work under different parameters than home subs. If you take an ordinary set of car audio subs like the rockfords and put them in an ordinary car speaker box and put them in your home it is going to sound terrible. Those boxes are setup and tuned to be used in a small space such as an automobile, not your home. Not withstanding, getting an effective amount of power to car audio subwoofers in your home is very expensive. If you did go this route you would need at a minimum a pro audio (commercial rack mount) amplifier and possibly a crossover (if the amplifier doesn't have one built in). Even if the amp has the crossover built in you're still going to spend over $200-300 on the amp alone to be able to drive such speakers.
I agree with what was said above, you'll have time to save up for car subs. By the time you have saved up your car sub money you may have even changed your mind about what kind of subs you want for your car anyway.
I'd recommend following
zieglj01's advice and get the Dayton.
Edit: The reason car audio amplifiers are so much cheaper per watt (without getting into overrated amplifier wattage figures) than commercial amplifiers is the lack of a need for a transformer. Voltage in car audio doesn't need to be addressed because it's already around 11-12v DC.