1.) Phrozen Sonic Mini 4k - best resolution desktop printer for small detail items
2.) Pretty rare to find second hand printers to be honest.
3.) You're going to need slicing software to turn 3D models into printable files and obviously 3D CAD software to make the 3D models. A slicer will be supplied with printer, but you can use
others, for 3D CAD you need to shop around for a package that works the way you want and is good for the kind of design you want to do. Fusion360 is a jack of all trades which will allow you some professional capabilities and is pretty easy to use though not necessarily obvious where the commands are or what they do for a newbie, Plenty of youtube help etc. Sketchup is great if you never want to go beyond 3D printing and has a straightforward UI: You literally just draw in 3D space and any flat surface you make gets filled with a surface. Keep going and link them all together and you get a watertight 3D shape, hopefully. SKetchup normally requires a little help to make printable files, but the info is all out there. Plugins can make it a lot more useful too.
4.) Using the printer is the easy bit, but there's still a learning curve with that. 3D design is a discipline with many branches that will take time to master.
As for laser I can't help with hardware, but you need either 2D CAD package or a Vector graphics package. Both have different approaches to learn. CAD is more rigidly defined by dimensions and mathematical functions, Vector graphics is free flowing and without constraints. I've always found the CAD approach preferable personally, even when producing web graphics I often do it in CAD!