Andy,
I powder coated my rear end, so to speak, because it was not that expensive. It did cause me big problems; I didn't prep it properly and so then had to spend ages removing it from the inside of various parts where bolts were supposed to go, etc. I am pretty sure powder coated will outlast paint, though. But that said, I guess it depends on whether you will use the car in all weathers. I've been so impressed with how you've done the underside; it looks like you could drive the car through the sea on a daily basis and it would never rust! It's got paint, though, not powder, and the bodywork is far, far more susceptible to rust that the axle and associated parts, so in effect you'd be spending more money to coat something that would never really decay and which will almost certainly out survive the rest of the car. It is therefore not logical in my view to coat the rear.
As for the rubber shown in the picture, I'd replace the lot. It's not that expensive, and if you don't powder coated, you'll save money.
James
