My home is wired for a security system. Any way to take advantage of this to integrate a security system into ST?

There’s a cloud to cloud integration with Simplisafe which is quite popular. Reading through that should give you a good feel for the integration.

Because SmartThings has an IFTTT channel, you can also get some indirect integration with any security system that has IFTTT, like Abode Security.

As far as your existing wired sensors, that just depends on the exact details. There may be something you can do with it, but it will probably require additional devices and some rewiring. So that may be doable, but it won’t be simple. Once you know what exact brand and model you have, just search the forums to see what people have done.