Tap and die set. Suggestions?

Thanks everyone for the advice. I think I'll get M4,5, and 6 separately. Am I right in thinking that you need a taper to start the thread and a separate finishing tap to clean it up. Also, what is a plug? Oh yeah, can you tap an imperial thread with a slightly larger metric taper to convert it or is it a case of drilling out the thread completely and starting a new one in metric? I hope this is comprehensible...

Wise. No-decent one will have tapered nose and straight body-all you need. Thread conversion as you describe is not possible-it will be a bugger's muddle of a bodge. You might be able to get away with it, but not a proper job by a long way. Plug is a thing you screw in to block up the hole.