For cutting aluminium at a sensible feedrate it's good to have about 12000rpm. You're correct in thinking a higher voltage motor outputting the same amount of power will dissipate less heat than the lower voltage motor due to I2R losses. Going above 24V gets very expensive since so called 'high voltage' ESCs are much more expensive than 24V (6s) ESCs. So ideally for 12000rpm you want a 12000/24=500kv motor. Much lower kv motors are significantly more expensive since they're rated for a higher power.

Arguably the best way to get the required RPM is to buy a motor with a suitable stator core size to get the power you require, remove the windings and rewind it to get exactly the kv you want. Although by no means trivial it's not as hard as it sounds. Sometime I'll try it. Depending on whether the stator is wired in star or delta (generally delta) you can change the kv up or down by a factor of square root 3. I did this on the brushless motor I have to act as 'electrical gearing' to avoid changing pulley ratios to obtain the required torque.

As has been already mentioned you require bearings which tolerate axial and radial loads at a high speed, which makes angular contact bearings the best choice. An additional reason for not using deep groove bearings, not often mentioned, is that they have a radial clearance. That means there is a gap, albeit small, between the ball and the outer race there to accommodate changes in temperature. This will clearly affect the general accuracy and run-out of the spindle, in addition to reducing the rigidity as there is no pre-load.

You can get 8mm bore angular contact bearings here for a little more than the magneto variety:
708 angular contact bearing
I think they should be better than the magneto bearings, although I can only back that up with visual evidence! Either way you would need to acquire a pair of bearings and incorporate a way to preload them. If you can find a double row 8mm angular contact bearing that'd be much easier.

With regards to the top bearing in your mount I agree with m_c - it's not required since the existing motor bearings will easily provide adequate support. In addition not having to bore the housing on both ends and maintain concentricity is a bonus.

Do you have a lathe? If so making a housing is straightforward (although it does require careful machining to bore the bearing housing with sufficient accuracy), if not then er...