If you don't ever plan to run more than a 1hp motor with the VFD, a Kbac-24d will be a little cheaper than the 27. If you think you'll drive multiple machines, or may want to use it on a larger motor in the future, I agree with using the kbac-27d. You can always drive a smaller motor on a larger capacity VFD, but you can't drive a larger motor on a smaller VFD.
FYI, there are cheaper options than the kbac series, but some of them may require 220vac, and normally they're not rated for dust/water tightness like the kbac drives are, so there's risk of shorting out the internals if you let the dust build up, or if you don't build your own enclosure.
The main thing you want to look for is that it's a single phase in, 3 phase out VFD, and that it's rated for AT LEAST 1 HP. (You can make a 3phase in/ 3phase out VFD work, but that's another can o' worms).
One more thing: If you go with a KBAC drive, you'll want to get water/dust proof glands for the cable connections if you're interested in keeping the NEMA rating. Another option is to just pump a little silicone around a regular connector, though it's not as tidy looking.
