In terms of adding equipment post-factory, nothing is impossible, but price-wise some items are really out of the question. Adding the rear headrest not only involves changing the rear shelf, but also adding the wiring for the front dash switch plus the appropiate wiring.
All of these involves removing a lot of panels and a lot of labor, which can be quite expensive. The last job I saw performed at the dealership on a W124 sedan ended-up costing about $2,500.00
Although, I would recommend adding the rear electric sunshade if you decide to add the headrests.
The headlight wipers might involve less labor, but parts are expensive.