Might be easier/cheaper to sell the C180 and buy another... the price difference between a W202 with electric windows all around and without shouldn't be massive on a second-hand car... assuming you have a W202 or W203 of course!
If not, a breaker might be your best bet... off the top of my head, you're looking at:
- Wiring loom
- buttons (front/rear)
- new button surround with extra holes
- door cards
- motor/actuators etc
For the hand-rest, that's not too hard; but it is a sought after mod, which makes the parts fairly hard (and expensive) to come by!
M.