Well, the little green Lohman Circe is what I started calling with, and I still use it. You can make a pretty good range of calls with it considering that it's a closed reed caller. I've never had good luck with a howler, but then again I don't howl much anyway. Open reed is the way to go because of the flexibility in sounds you can make. If turkey calls are working, try those too!
With mouth calls, I usually call for a minute and sit for two, then call again. I don't think it makes much difference but I know that I don't have enough wind to call continuously. If using a digital I let it run. Calling session length is determined by how much area I have to cover. In Arizona, where you can make as many stands in a day as you have a mind to, we call 15-20 minutes if we feel like moving fast. At home in Missouri where my calling spots are few and far between, I call for a half hour minimum.
As far as when to call, any time you have time is a good time! I myself usually only go after them during the winter months so that I can get the prime fur, but they can be called and killed all year long if you've a mind to.
You're on the right track as far as calibers go, but if I were you I'd also check out a CZ rifle before you spend money. I bought one used from a buddy and now I'm completely sold on them, damn accurate rifles. Point is, look at all your options before you buy.