If the rear electric windows are slow it's due to lack of use. Put them up and down a few times a day and they'll recover. With the sunroof on tilt they allow for excellent ventilation by the way. Water leaks in the boot can be caused by failing tail light gaskets, the inner ones cost 40 quid each. Old BMWs tend to suffer from a bit of condensation in the boot.
Control relay is hidden behind the indicator inside the inner wing.
A body pack with rear apron and sills isn't uncommon, personally I don't like it. The Six has a very big heart, the wonderful M30 engine. The 2.8 uses Jetronic-K injection and gives 182bhp. It's alright but you really want the full 3.5 for a higher number of grins per mile. This does away with points, vacuum advance and all other such mechanical aids and uses a micro-processor to decide when to fire the coil and injectors.