I can tell you that a .8kw spindle will absolutely be more than adequate for cutting and shaping Alu, as seen by the shot of the new spindle mount I was machining tonight out of 100 x 100 x 32mm alu.
I used a 6mm x 50mm 3 flute endmill cutter at passes of 0.5mm deep and had my 800w spindle VFD set to 400Hz. Probably too fast but i'm still getting used to the spindle and VFD calculations. I initially had the (mm) feedrate set to 120 but bumped it up to 300 for a bit before dropping it to 240, and it seemed to cut smoothly without any racket.
I will say though that your machine needs to be absolutely ROCK SOLID, I can't stress that enough. You also need to really clamp it down well and you'll get a great finish. If you have any slack, or anything in the design that allows even a minute amount of vibration, you'll be shocked at how machining alu greatly amplifies it, and makes machining it much harder on the machine, the motors, and the cutter especially. I was slack tonight and didn't have suitable clamps handy so I just used double sided tape on the bottom, and screwed 4 roofing screws (2 along 2 sides) to help stop it moving.
I didn't count on the tremendous heat build up, that pretty much melted the tape, and the block then slid to the side on a sideways cut and stuck, breaking the nice and shiny, brand-spanking new cutter! The slot was cut first, then the 43mm circle was cut to a depth of around 21mm before the block slid, but by then the block was very, very hot to touch, I have quite tough skin but I couldn't keep my hand rested on the block for a period of time, it was just way too hot. Cooling will be a major problem for cutting alu.
So there is definately an art to working out the correct VFD setting, cutter to use, and feedrate to maximise cutting while minimising heat build up.
My steppers are NEMA 23, 3A rated (i'm guessing around 220-280oz-in?), and the crap TB3650 controller that came with my machine was replaced with a Gecko G540, and as I found, the very obvious running and torque differences between them are unbelievable!
I would very strongly recommend getting a G540, especially if you are going to push your machine by cutting alu.