Do you want a laser to mount to your CNC or a standalone diode engraver? You won’t be cutting metals with anything short of a 1000w fiber laser whatever the advertising says but you should be able to mark stainless steel if you go slow and use a marking compound.

I’ve heard good things about the Sculpfun S9 and the XTool D1. Generally speaking, you get what you pay for. It’s worth spending money on an air assist as it will make the laser far more effective. Check out the “unofficial”groups on FB for any machine you plan on buying - that should give you an idea of the issues and successes people have with their machines.

Check out r/lasercutting as there is some good info there. Realistically you may be better off with an entry level CO2 laser than a diode - especially if you plan on making products to sell.