I should probably revise my statement slightly, or at least expand upon it... There ARE chassis you can buy in the $500-$600 dollar range. Most of them will NOT come with a motor, or if some how they do, it won't be much of a motor. At the very least it will be single speed, and probably open frame as well.
You can get some decent bolt together kits here or there. The GIB by Polar Bear Forge is still being offered I believe, for about $275. You have to provide your own wheels and motor, and some nuts and bolts, etc... as well as do a little drilling and tapping, IIRC.
There are a lot of options for wheels on eBay these days for not a lot of money, and you can get a used 3 phase motor and a chinese VFD for less than $200 if you shop around.
The kadrmas grinders don't look terrible, though they're a relatively new company. They do seem to be priced very competitively though. Again, you'll have to add your own motor.
If you have a scrap yard or a fab shop near by, you could probably buy some scrap tube and put a frame together for about $100 to $150. Buying all the steel brand new might take you closer to $200 or $300 by the time you got all the little springs and knobs and whatnot.
As Laurence pointed out, the Grizzly is always an option, though it is single speed and kind of a one trick pony. I'd personally spend the money on a tool arm style grinder that you could expand as you get more into the knife making, or get a few more dollars saved up.
So, long story short, there aren't a lot of options for $600 unless you scrounge and build. Double that budget, and you'll have several options from a number of grinder manufacturers, along with an extra accessory or two. Double it again, and you start to get into some professional machines that will really shorten your learning curve.
