Every workshop is different, so rather than relying on general advice, it's worth getting your own set-up properly assessed. A system that works well for one shop can be quite wrong for another with different ventilation, floor space or process mix, and an on-torch extraction option, such as those available for Fronius torches, suits a different layout to a fixed overhead hood.
Switching speed, the time the filter takes to darken once it detects an arc, is worth checking against how you actually work rather than assuming faster is always better for every budget. For most general fabrication and repair work, a mid-range auto-darkening helmet with a sensible shade range covers the vast majority of jobs comfortably.
Plasma cutting uses a jet of ionised gas, usually compressed air, forced through a nozzle at high speed and heated by an electric arc to a temperature hot enough to melt through electrically conductive metal. The molten material is then blown clear by the same jet, leaving a narrow, clean cut. Unlike oxy-fuel cutting, plasma works on any conductive metal, including stainless steel and aluminium, not just carbon steel. Hypertherm is the plasma cutting brand we get asked about most, and it's worth understanding the basics before comparing specific units.
If you're not sure which supply a particular machine needs or whether your workshop can support it, it's a sensible question to put to a supplier before ordering, and the team at plasma cutters can talk through the options against your existing set-up.
Abrasive discs look interchangeable on a shelf but perform very differently depending on what they're made from and what they're used on. Cutting discs are generally thin, designed to slice through material quickly with minimal heat build-up, while grinding discs are thicker and shaped to remove material from a surface or clean up a weld, rather than cut all the way through it.
Shade range and sensor count are the two specifications that matter most in daily use. A wider variable shade range lets one helmet cover a broader spread of processes and amperages, from low-amp TIG work through to higher-output MIG or MMA, without swapping cartridges. More sensors generally mean the filter is less likely to miss a low-angle arc or a strike caught at an awkward viewing angle, which matters most for TIG welding where the arc can be harder for a sensor to pick up cleanly.