quickcanary,
All of my SIG's are the alloy framed guns. I havent encountered any frame cracking or wear, other than normal wear, with any of them, and I usually shoot only full power rounds, or equivalent reloads out of them. All have quite a few rounds through them at this point.
Corrosion has never been an issue with any of mine, newer or older. I've never seen rust of any kind on any of them.
The P220 isnt a "large" gun, and is very close to the 1911 Commander in size. I used to carry a Commander and switched to a P220, and then to a P226, and finally a P229, the later two , both 357's. All SIG's, regardless of model, (P230/232 aside) are very close in size, and really only vary about half an inch here and there.
Not sure about the sights, as I dont have any CPO's. Most of the used SIG's I have bought had night sights on them. Either way, they are easy to swap out yourself if you have the tool, which is a good investment if you have SIG's. Its very easy to use, for both installation/removal and sight adjustment. So far, I've sold the old night sights I removed on EBay for about half of what a new set costs, so it brings down the cost even more.