Hello
The attachment point on the frame are:
1.axe for rear swinging arm
2. Between fuel tank and seat
3.on the front support bearer (don´t know if it is the right english word for it).
And there is an additional stabilizer on the rear swinging arm.
And it is no problem to drive an ES 250/2 with sidecar. It was fitted this way from the factory, so you cannot compare it with the BMW
But you need to change several parts on th bike:
Front swinging arm
Rear swinging arm
Additional steering damper
sprocket on secondary chain drive
Speedo drive
By the way:
The bike on the pictures is for sale from a friend of mine.