what are you cutting? the heads are for two different tasks (mostly). the K2 unit may not be rigid enough for the type of (relatively) heavy milling the Taig unit can do.
however, if you are just milling the same stuff that the bosch unit was doing, you probably can slow down the feed to match the speed and mill just fine. depends on material and tooling though. again it comes down to what you are cutting.
you can get a taig spindle, they do rock, but the heavy motor cantilevered out and off-center the way it is may not do your K2 Z axis any favors if it wasn't built for it. a lighter motor may be in order. if you want a taig spindle i have some er16 spares from when i went to vfd spindles myself.
thats another option; just going to a vfd spindle yourself will cut the awful noise of that colt while keeping all your speed if that is an option that fits in your budget.