It depends what you mean by "natural limit."
You can do really odd things to yourself with pumping, so it may be a matter of taste - that is, when it’s not a matter of safety.
However, I’d recommend that you heed petitfaun’s advice and check out The Pumper’s Forum section around here. Avocet8’s thread in particular.
As for the product advice:
Check out Boston Pumps online. They give you a decent product at a decent price - for me it’s all about price these days: cheap is real. So I’d recommend buying a cylinder from them, and I’d also recommend the silicone sleeve called Sleeve Master .
For the pump itself, as you are a newbie, it is recommended that you get a good brass pump with a gauge. Most of the "penis pump" manufacturers jack the price up and have unreliable quality. Even Boston Pumps - though better than most, they still sell a plastic gauge pump for too much money.
I recommend this instead: Harbor Freight brass pump with a gauge. I have one of these and they are pretty durable and accurate.
Once you have a decent amount of experience with pumping, a brass pump from Boston Pumps or some such without a gauge (like this) will be fine.
Before: I'd like to show you something I'm very proud of, but you'll have to move real close.
After: I\'d like to show you something I\'m very proud of, but you guys in the front row will have to stand back.
God gave men both a penis and a brain, but unfortunately not enough blood supply to run both at the same time. - Robin Williams (: