Plyzo,
Please fill in the basics on your profile so we tell where you are from.
In answer to your questions:
Aluminum can be powder coated. Powder can look very good if the surface is smooth, but it is way more expensive than paint. More durable too but for those of us that change our minds a lot, it's not always the best idea. Have you completed a dry build to know for sure that you will not need to add, subtract or modify anything on the frame?
The trick is to dry build and get it running and modify things that didn't work teh way you planned it, and when you are satisfied with the whole package, strip and paint/powder coat it.
Lower triple doe not need to have the stem machined out or pressed out. I usually hit those with a rattle can. Cost about $2 (1/4 can). I have tried rattle cans, powder, spray guns, professional paint and I come back to rattle cans. Client build might get a pro paint job bit race bikes don't and neither do my street bikes.