Any advice on buying taps. I need sizes from 2.5mm to 6mm (including
3.5 mm - even though my local DIY shop tells me metric screws only come
in whole numbers) for tapping mainly aluminium and occassionally mild
steel.

MY internet search seems to find huge sets of taps and dies at
suspiciously low prices or individul items at pocket wrecking prices.
Materials seem to be carbon steel, tungsten steel and high speed steel.
Which is best?

Dave