I,ve been a tech for 25 years and I get by with 5 metric taps and dies/2 spark plug chasers/a thread file/set of metric drill bits and a pitch gauge. They are all decent quality (Dormer) and I use a variety of drives like tap sockets/ratcheting tap wrenches and my dies are hex so I can use sockets/spanners on them. I went this route after using my bosses Snap On set (the taps and dies were good quality but the drive tools sucked) apart from the few metric sizes we actually used in that set, everything else was useless to us.