I very successfully DJ'd with 12" 2-way boxes over single or dual 18" subs. Many times just I just used one 12" over a dual 18", running mono. Space in bars was always limited, and this did the trick. I just never liked the sound of a 15" trying to be a midrange driver - no clarity. Also, getting 12-inchers up on a stand, or on a pole over a sub, was a breeze.
Get a very good pair of 12's and one very good sub now. Buy the 2nd sub later, when necessary. Find some owners (here or elsewhere) to get a demo before buying. When auditioning any speaker, only listen to ONE of them, as you can be tricked, or misled, by the sound coming from 2 places. Trust me, they will both sound the same.
Subs: I never leave home without one.
If you will never buy subs, then get a powered 15". The lowest end that I can recommend that sounds very good by itself is the EV ELX115P. I'm sure there are others, and prices go up from there.
The ELX118P, while functional, never really gave me the "goosies" - there are better choices in powered subs.
ALWAYS use a high-pass filter (about 35-40 Hz) on unpowered subs (powered subs have this protection built in).
This is my personal opinion, of course.