I have a 2009 MK7 ford fiesta with 100K Km mileage which is sitting in my garage most of the times and I drive it quite gently. 

Recently I've noticed some small rusted area (like 3-4cm) at the base of rear springs where the coating plastic is ruptured.

So I'm wondering if I should change the springs immediately to avoid sudden failure, or I can carry on using them until they fail?

And if I have to change them should I change all the springs altogether or just the rusty ones?

Should I look for only OEM springs or any spare springs are pretty much the same? If they are not the same what are the better brands?

What is the expected cost of changing all 4 springs with the best available spare parts?

Thanks in advance for your tips. 

If a spring does fail then I would always change the opposite side spring as well to ensure that the car will sit level as there is no guarantee that the other spring will not have sagged with age.

In its life we had to change both front and rear springs on our 2005 Fiesta. When the front one broke we needed to get the car lifted and taken to the garage but when the rear one went it was the bodyshop that noticed it when it was in for a tart up - my partner had been driving it with no awareness that both springs were broken. I've also had both front and rear springs break on my 2005 Mondeo but in both cases the failure was immediately noticeable and the car was slowly driveable to the garage - but I would not have gone far, nor on a fast road or motorway.  I guess the takeaway is that old springs fail but in a manner that may or may not make the car unusable.

We've always had Ford springs fitted but that's because we use the same Ford dealer I've been going to for more that forty years!

using H&R lowering springs on my mk7..when you get them done ask the garage to check the rear rubber mounts that go on the top of them as they wear out.