Welcome on board! You will love lasers!
1) Usually laser can cut SOME materials only. And CAN NOT cut other materials. So if you have chinese laser with CO2 tube - you will be able cut wood, plastic, paper and some other materials BUT you can not cut metals - steel, aluminium and etc. So usually table in laser engraver/cutter is made from aluminium.
2) Focus is setting based on top of material. Be careful with cutting of thick materials. Usually you will cut this materials in few passes. And if for the first pass you must set focus on top of material, for all other passes you must set focus to the top of material BUT AFTER setting focus manually move table slightly up (depend from how deeply was cut during previous pass/passes)
3) You couldn't do real 3D on laser (especially on Chinese with glass tube). You will have (I name it) pseudo-3D. Gray scale image will be engraved based on machine's settings POWER/SPEED for each level of grey color. Together with problem that all materials are not homogen final result (depth of engraving in certain place) is difficult to predict.
4) I am very familar with GCC laser engravers and they have stepper/servo motors and belts for moving laser head.