Few meet this requirement. I have the same desire, and now have 3 Sculpfun machines, s30 Pro, SF-A9, and iCube Pro Max, in that order. A plus is Melvin Isken’s ( @misken ) superb website dedicated to Sculpfun machines (https://diode-laser-wiki.com/).
It is better to build your own enclosure setup than use one ready-to-go. Most have no fan, or a lame one at best (greenhouse duct fans work great). You do not need to build a box using overkill like I did. PVC pipe and canvas sides would work too.
I recommend a 10w machine as the minimum. You say 3mm now, but…
The “cube” machines are difficult, but most rotaries use the same stepper motors as the axis drives. The only issue might be getting a working hookup cable.
Depends. My metal marking is done by using Cermark Ultra spray. Very expensive, but it works like a charm. You can also buy an optional IR module ($$$) that will mark metals directly.
Suggestion: Don’t take my word for it. Search the forum for the various machines and read about the reported issues before you spend the money.
In the image, left is the S30 and right is the SF-A9 set up for longer-than workspace use (the S30 sits on a riser).