One thing to beware of: you say the house is wired for "German power" and "US power". I assume this means you have a combination of the typical receptacles for US 110 volt circuits, and the typical receptacles for European 220 volt circuits. That probably does not account for differences in frequency of the AC power. US is a 60 Hz system, while some of (maybe all?) of Europe uses 50 Hz. Induction motors will operate at a slower speed at 50 hz than at 60 hz, which can affect performance, and in some situations, cause the motor to overheat. I would avoid buying US / NA tools to use. Use local machines and sell them when you leave.
Not sure why I was thinking the joists were perpendicular... they're parallel to how the wall will go. Luckily, a joist runs right down the line (about an inch off of the original plan) where I want the wall.
One thing that I know I need to do is to check the insulation in the attic above the garage. It doesn't get as hot in there as our last house that had ZERO insulation in the garage but it definitely gets warm in the summer. Winter doesn't seem to be much of an issue, though. Can I just put batting above the room or am I much better off paying to have insulation blown in if necessary?
Thanks! I find this very enjoyable, as well. It's been a long time since I've had the opportunity to build an enclosure and controls. It was fun designing the circuitry for the relays and related components and coming up with my own latching circuit and all that stuff.