What specifications must I consider for a DC motor?
The short answer to your question is that you start with the specifications you need - speed, torque, etc. - and then from that subset of units, you pick the one you can power, pay for, and fit where it's needed. If the last three elements don't fall into place, revisit the initial spec to see if it makes sense, and if it can be massaged into something workable. Sometimes the thing doesn't exist yet - sometimes it can't. Play nice with mechanical engineers and physicists, and they'll tell you what's impossible so you know where to stop.

Engineering is sometimes just a highly specialized branch of applied economics.