I got the impression that I'd need either massive amounts of aluminium or preferably a welded steel frame for a raised x-axis to work. Neither would work particularly well for me (since I can't weld, nor work steel), so I think I'll go with this gantry design. I wont be doing massive amounts of milling, just the occasional part.
I'm one of those people that build mostly for the sake of building, not really with any clear goal in mind.

What kind of doc and feed speeds do you think I could expect from this? Something like 0.5mm and 1000mm/min, doing alu and still getting good finish and precision?

As for the aesthetics, I completely agree with you. As a matter of fact, I intentionally left that extrusion end open to have a fun first project to route. I would have liked making the rear end cover cover more (but not so much as to bump in to any high parts I'm cutting), but I'm working mostly with standard lengths and parts, meaning this was way cheaper and in my mind a small cosmetic tradeoff...

Thanks for the tips, I'll probably start ordering next week, need to make sure I'm completely decided first. The building will drag out a bit until the summer is over though.