Does site designer 5 come with the ability to create forms? Or is that a separate purchase?

It has the ability to creat forms but you have to do the back end scripts etc yourself or mess about with the WFB files same as you have to do with WFD

If the form you want to create is kind of simple, I have a sample form with a back-end script here: https://mock-up.coffeecup.com. Just select your framework.