Rust spots are usually around the front turn signals/fender area.

Also front fender, area closest to the front door tends to rust as water and debris is kicked up here.

Same with back fenders, areas where water is kicked up.

Floors, you must watch that water has not leaked in through a leaky back or front windshield, or a sunroof if applicable.

Bottoms of doors sometimes like to rust as well.

I would be leary of a few things on a 617NA transplanted into a 240D. Engine mounts first off. The way in which the 240D starts, and the glow plugs operate (use of a pull knob) could possibly pose a problem. Rear end ratios would be different on the cars I believe. Other than that, if its a clean transplant, and is rust free, its a pretty good deal.