My laser uses stock board and has been very reliable, I do not do many engraving jods (5%) the k40 with air assist and using corel draw is a very capable unit ,I admit I am thinking off a better machine at $2500 for a 60 watt 400mm x 600mm cutting bed,only for the quicker speed and thicker cutting capacity, I need bigger cutting bed for very few jobs
Do not be put off using the k40 to make money. Just be aware that it took me 12 months to learn the machine and software.
Engraving glasses will require a rotary attachment and uf I was you I would wait on that until you learn more.
@Phillip_Conroy sounds good, I just need to get it working again. With your amount of work you’re doing on it, you could justify a new toy! 
If you do buy a k40 ,get air assist straight away,and keep a logbook of how you do stuff.their are a few traps for newbies like selecting the correct board in the laser cutting software, and making lines 0.001mm thick so that the machine only cuts the line once .oh and the cutting bed is crap.the clamp is next to useless .I made my own pin bed out of 10 mm long magnets with screws ontop,I only needed 12 to do most jobs .the laser beam will only cut at the focal length plus 2 to 3 mm ie focal length is 50. 8mm plus 2mm =52.8mm aim for the center thickness to be this distance from the bottom of your focal lens