Hi Sus,
this is pretty much the simplified speed chart that I use.

You can see there is a hell of a range from 800rpm with a 20mm HSS tool in mild steel up to 160,000 for a 1mm tool in Aluminum.
Having one spindle to cover that range is impractical but having a spindle that can cover say 1000 to12000 would get the majority of it.
Clearly a spindle that cannot safely go lower than 10000rpm would severely limit your ability in steel, even if it were OK for aluminum.

Craig