The SLG3000 only accepts 31khz (VGA) input. Won't work by itself with a PCE. If you don't care about arcade pcbs, just get an XRGB-2plus. As it's typically the cheapest. The 3 outputs in HD, but it's Japanese HD. So, a lot of USA TVs will not work with it's odd 1125p output. Also in HD mode you can't use the scan lines anyway. The only reason to buy a 3 over a 2/2+ is if you are using a console that outputs in 480p (Gamecube, Xbox, some ps2).
You are still going to have to mod the PCE though, so you might as well just mod the it and use your commodore monitor. 15khz RGB CRT will never be beaten.