On metric bits and taps, I've been turned on lately to the existence of form taps, and I really like the idea. They're stronger and less likely to break and form a stronger thread. The downside is that they require odd sized pilot holes, somewhere slightly smaller than splitting the difference between minor and major thread diameter.
I used to really want a metric drill bit set, as I pretty much exclusively work with metric fasteners. In practice, though, I don't know that a set of whole and half metric diameters would be helpful. A clearance hole is always three or four tenths over, and a tap hole is always somewhere two or three decimals down the rabbit hole. When searching for the optimal hole size for a metric form tap, sometimes a fractional bit is closest, sometimes a letter bit, and sometimes I just have to bite the bullet for a metric decimal bit.
Take M8 for instance. Close fit clearance drill is 8.4 mm. That's .331". Closest letter drill is a Q at .332". Closest fractional is 21/64, which would be tight at .328". I think I ended up buying a letter Q because it was less expensive than an 8.4 mm. When it comes to the M8x1.25 tap drill, you're looking for a 7.53 mm bit. These exist, but they're specialty and expensive. A letter M drill is 7.49 mm (.295") and is much more common and affordable. A 19/64 fractional is also common and comes in at 7.54 mm (.297"). I chose the latter as my tap drill.
Long story short, if I were making a metric drill bit set and trying to be frugal, most of the bits wouldn't be metric! There's usually a fractional or letter bit that's within a thou, which is plenty close. I've also recently discovered the amazing cost-per-performance drill bits that are the
YG-1 Gold P line. Instead of piecing together individual fractional, letter, and metric bits to make the perfect set for common metric fastener sizes, I recently just bought a full 1/16 to 1/2 fractional set, as it was an amazing value. This is just the general shop set, so hand tapping. There'll be a bit for tapping any metric size there that'll be close enough for hand tapping.