That's a good price. I bought a P229 DAK in .40 SW for a little less, but DAK actions are an acquired taste.
You can run 357 SIG through a .40 SW magazine, but not the other way around. Otherwise, all you need is a barrel change. I've used it, but 357 SIG is too rich for my pocketbook, and it's hard to reload and maintain the correct shoulder length.
A 229 the mags are exactly the same. A 229 you can also use the longer 226 mags in it, they will just stick out the bottom but again, the 40 and 357 use the exact same mags in 226 as well.
You are thinking of the single stack P239. That is the one that has 357 specific mags, no such animal on the double stack 229/226.