If you're playing small venues, I'd avoid 18" subs. You won't need the relatively small amount of bass extension they might give, they will be cumbersome, power hungry and will likely muddy everything up, especially in smaller rooms. If you are using a sub, 10s in the tops and 15s, or even 12s for bass will be more than adequate in small-medium sized venues.
Quality is what counts with subs, not driver size. I have a 12" sub (this one - https://www.soundonsound.com/reviews/fohhn-xs22) that out-performs many 18s I've heard. Two of them has proved more than up to any job I've been faced with.
You might like to look at some of the powered sub plus mini line array PAs. Portable, very clean sounding, simple to use and fill a room remarkably well. Not generally cheap (avoid those that are). However, if you plan to buy once and cry once, worth considering.