If it's in your budget, I would just get a pair of cheap shoes for spinning -- if you shop sales at Performance, Nashbar, etc you should be able to pick something up for well under $60. Put SPD cleats on them, and get used to clipping in and out on the spin bike.
Then when you have your road bike, and you are used to how it feels and fits, take some time and try on lots of shoes. Fit and sole stiffness will be much more important on your real bike, which you will probably spend much more time on at a go than a spin bike. Then talk to your LBS about different pedal systems -- they're usually pretty helpful about pedals.
Lots of people do run SPDs on their road bikes, and are quite happy with them. But I wouldn't let the fact that you want clipless spinning shoes determine what pedal system to use on your bike, at least not without doing a fair amount of research first.
Good luck!