All good points.
To answer your earlier question about older Vespas, the P200E as pointed out already is really your only decent choice for a couple reasons- It's the most common of the older Vespas you'll find, it has the biggest engine they offered and with 12 volt ignition and battery can actually be a reliable scooter, it's boxy looking and therefore ignored by the majority of the vintage scooterists as not collectable, so the prices are lower than nearly all of the other models. Oh, and all parts are still available from a bunch of suppliers.
Downside - it's still a scooter with a 200cc 2 stroke engine that will have to pull you, your sidecar and whatever else you're hauling. Your range of use will be very limited.
I love the idea of putting a hack on one of your BMWs by the way.....