I use
this crimper from moddiy.com, which has served me well for a few years of light use. It's even tolerated some real abuse such as crimping far too thick wires (two 18AWG wires twisted together - roughly equal to 15AWG - in jaws meant for 16-28AWG) and oversized crimp pins. Don't think I paid quite as much for it as it costs now, though. Also planning to get some replacement jaws for larger gauge wire, but that's not for normal PC use. 16-28 should be perfect for the majority of PC wiring needs.
Other than that, I have an automatic wire stripper from somewhere (
looks mostly like this, but not bought from AE), cheap edge cutters/flush cutters from AliExpress, and a
ModMyMods Billy Brown's ATX Pin extractor tool (originally bought a cheap set from AliExpress here too, which broke within 20 minutes of first use. Fear of impaled fingers led me to buy something better). Terminals and connectors can be bought from many places, I've used Moddiy.com before as they have good pricing and a well sorted selection for most PSU makers etc. If you're only shortening the cables on the "far" side (not the PSU side) you don't need new housings, but you need crimp pins.