First of all, you have to pay attention to rigidity. Even the most nicely-looking engraver but with mediocre rigidity (e.g. twisted aluminum profiles) will not ensure proper operation. This can cause jagged edges, burrs, and clear traces of the tool vibration during operation. The machine can not have sliding elements because they will quickly abrade and backlash will occur. So linear roller bearings and ball screw gears must be applied, which will allow the elimination of backlash.
As for the control system, engraving machines which communicate with a PC via serial or parallel ports have already gone out of date. For large works, in particular 3D ones, the file can reach tens of megabytes. Currently only Ethernet and technologies based on it come into play, which enables convenient operation of the machine with full visualization while working in real time.
A highly important thing is also the control software. Forget the printer driver which allows "printing" to the engraving machine. The engraving control software must generate the tool path itself (internal and external correction, concurrent and counter treatment, excavating pockets, excavating pockets with detection of islands, working on many stages of plunge cutting, cutting corners with an engraving cutter, etc.)
Well, and finally the speed - the fastest engraving machines reach the feed rate up to 1500mm/s, but the speed 150mm/s is good enough. Machines which reach speed not exceeding 50mm/s are toys.