When it comes to hand tapping difficult materials I'm impressed by serial taps. The cutting goes easier and less risk of breaking things.
No comprehensive sets around so best to buy when you need as they aren't cheap. I get mine off E-Bay from DavetheTools over at Pulborough. He sells the Vogel brand.
Realistically the sensible option is probably to get known brand mid range taps for the main set and supplement with serial ones when something difficult comes along before you wear out your general purpose ones. Thats more or less what I'm doing in supplementing my decent good collection of mid range ones although, given the choice, I power tap with industrial rated breeds.
Concentrating on comprehensive sets rather than what you actually need can leave you with alot of (expensive) unused tooling.
Clive