At some point in the next week or two, I'll want to drill and tap a
dozen holes into stainless steel tube, this is 2mm wall thickness, 42mm
diameter handrail, I'll have a 100-200mm offcut to practice on.

Now, we didn't do metalworking at school, so I'm lacking in the basics,
but a bit of searching seems to suggest ...

For M5 bolts, use a 4.2mm drill, assuming it's 0.8mm course pitch (need
to check) hopefully an 18V cordless drill/driver isn't going to struggle?

Drill slowly to avoid work-hardening

Lubricate (use WD40 at a push)

As I'm tapping through, rather than into, the steel use a spiral point,
rather than spiral flute, tap.

What flavour of HSS (cobalt, vanadium, extra vanadium, nitride/oxide
coating etc) does the tap need to be to cope with 304 stainless?

I see 1/4" hex shank taps are available, e.g.
http://uk.rs-online.com/web/p/threading-taps/6699384

Will these work OK in the drill/driver? Or should I hand tap?

Given I'm unlikely to ever want/need a full tap/die set, can you buy
screwdriver or T-handle style single taps?

Any other tips appreciated.