The questions you need to ask yourself are 1) how big will my projects be, and 2) how much engraving will I be doing? If you will primarily be doing cutting then any machine will work well including your LR4. The speed of cutting will be limited to the power of the laser module. If you are focusing on engraving, then the mass of the machine plays a big part in how fast you can engrave. For engraving, you’d be better off buying a machine rather than building.
For example, back when I was engraving white tiles, this topic about $79 laser engravers came across the forum and I bought one. I replaced the cheap laser head that came with the machine with my NEJE 40640. The time to engrave a 4" by 4" tile corner to corner went from around 80 minutes on my primo down to around 55 minutes on the $79 machine. If I tried to engrave faster on my Primo, artifacts would ruin the engraving.
The $79 machine was a limited-time deal, but there are cheap (new and used) laser engravers out there. The $79 machine has a small working area, but, because engraving takes so much time, I rarely try and engrave anything larger than a 4" x 4" tile. I can still use my Primo for larger laser cutting projects.