Hi guys, first post here. So I just brought a 2009 fiesta style+ and judging by the previous MOT it needs new 2 new rear springs due to corrosion. Now I've come across Fiesta ST springs from a mk7.5 2015 model that were very reasonably priced and have a few questions since I'm a total newbie at these kinda things:

Are the springs compatible with my current suspension? Will they fit without too much hassle?

What will they do to the ride height of my car? The ST line are obviously lowered from factory, but from what I understand they're stiffer and designed to handle the larger weight of the engine in the ST, whereas mine has a lighter 1.25 petrol engine, so could it possibly ride higher at the front than the back?

Thanks in advance for your help!

You could fit the ST springs to all 4 wheels but not just to the rear as the car would then have a rearwards rake. You might need to replace your dampers though as the ST springs are about 20-25mm shorter than the Style so the dampers might be too compressed at rest. 

As you said the ST has a heavier engine, so has springs to compensate. It would get lowered, but it wouldn't look right, and you could sacrifice comfort. I wouldn't bother with them personally, and if you're after lowing get some Eibach springs online. How much were the ST springs? I bet the Eibach ones are a similar price. Aftermarket springs like these on their own are reliability cheap for a decent car mod.