I have a Kim Laube 521 Mini Lazor Clippers 2 speed. They do an awesome job clipping thick coated breeds or really matted dogs. They also do a better finish job, I find, then my Oster clippers. The only problem I have with mine is it gets the blade hot really fast. Not sure if it's because of the speed, it has a faster stroke per minute then other clippers.
I purchased mine directly from the Laube company. If the clippers that you are looking at are second hand, I'd be a bit leery of them. I guess it would depend on why you're buying it. Is it for grooming dogs professionally or is it for your own dog? The reason I'd be leery buying a second hand is that with lots of use the motor starts to slow down and doesn't perform as well.
My next purchase would be an Andis clipper. I've heard great things about them.