For your first laser, buy a laser with a power of at least 40 watts. I think air assist is also a must. Other than that, most features are bells and whistles, like that stereo in a new car. These features may be great, but the heart of the machine is the power the laser can produce. This is what makes money. A machine of less than 40 watts may be a good second machine for those who know exactly what they will be doing with the laser, but for newbies, I recommend at least 40 watts. I use both 40 watt and 50 watt lasers in my operations, and while I like my 40 watt laser for the speed and power it produces, there is almost nothing I can't do just as well with a 40 watt machine. My 40 watt machine doesn't have air assist, my 50 watt machine does. This dictates that all rubber stamps and woodblock prints, as well as most cutting projects, must be done at 50 watts. Not necessarily because it has more power, but because it has air assist. Most new lasers either come with air assist or are available as an option. You don't need air assist to do these types of jobs; it just makes them better, easier, faster and cleaner.
As for the physical size of the engraving bed, the bigger the better. However, with that came some tricky questions. How much space do you have and how deep are your pockets? My machine is a 400 x 400mm machine, there are only a few times when I need a bigger machine. Even so, it's used for rather odd jobs like carving cabinet doors.
Again, I can't really say how "you" will make money with your laser, but I can share my experience of where to find the best money, highest profit, and least problems. Your experience will vary, but maybe what I say will give you a new perspective, or turn on some lights in your mind and help you identify a new market.
Can you make money with lasers? Anyone who owns a laser and doesn't make money with it is either seriously misinformed or just didn't try. These words are heavy, but I believe them. Anyone who puts in the effort, learns the necessary skills, and a little bit of luck can make money with a atomstack laser engraving machine.
Over the years I've served just about every market, from making cheap little trophies to making parts for jet fighters. Some work just falls into my lap and no matter how hard you try, you can't replicate it.