I know it’s hard to resist the urge (I should know, I have accute gear lust syndrome) but I will say it again: don’t buy all of this just now 
- aluminium bed: yes, they cost a lot, especially so for an XXL. Definitely postpone that upgrade until you know you need it (in all honesty, chances are you won’t), in the meantime just reinforce the center of the wasteboard to avoid sagging (here’s an example, but search the forum for other ways people have eliminated that problem on their XXL)
- HDZ: probably a sensible upgrade you could do upfront if you are going to cut a lot of aluminium. Note though that since it is tougher than the stock Z axis, it is also less forgiving (i.e. if you mess up the toolpaths or zeroing, and someday you will, the HDZ won’t be damaged, but the endmill and/or the wheels might )
- Suckit for HDZ: note that if your are exclusively going to cut aluminium, a dust shoe won’t be of much help, its primary use is to deal with wood chips.
- Mister: search the forum and you will find tens of links to cheap (~20$) mister heads. But that is the easy part. You will then need a compressor to generate enough airflow & pressure to use it. I keep hearing about California Air Tools models here on the forum, they must be doing something right.
- Spindle: that is an expensive but also slightly difficult update to be doing. I recommend you search for “spindle upgrade” here on the forum to get a feel of what this means before committing to buying anything.
- Endmills: for aluminium, you want ZrN coated endmills. Carbide 3D store sells great ones, look for the one with “Z” in their reference (e.g. #278Z). I think a couple of single flute ZrN 1/4" square endmills woudl be most beneficial to your usecase. Then get a 90 deg Vbit to do these 45degrees chamfers.