Welcome, try not to get as addicted to buying plastic as I did.
If you come from an ultimate background, I assume you have fairly clean form. I believe that a Birdie would work well for you as a putter. As for midranges, try a Z Comet. Z is the plastic. They are very neutral, and will probably be very easy for you to pick up. And for a driver, I believe a Leopard is a good choice. They are probably the best first driver made. You should find it significantly longer and faster than the Comet, but still very controllable. These are all great discs to begin with. As you progress, you may want to add something more stable to your bag. Rhynos, Buzzzes, and Teebirds are discs that are a step up in (over)stability and should enable you to do some different shots when you have begun to get the hang of your starting stuff. If you are concerned about weight, I would ignore the common advice of beginning with very light stuff. I would try your putter and midrange at about 175g, or slightly less, and your driver at 170, or slightly less. If you are a healthy male with an athletic background, you should not have a problem with these weights.
Anyway, I hope this advice was helpful. Others may differ, and they may also have good suggestions, but I'm confident that these are the sorts of discs you're after.