Most Cat. 1s will fit, though some of the largest sized U-bars may need to be mounted by opening turn buckles to max width then lifting lift arms to mid-height where you can get full spread to mount the U-bar. Such is the case with my Greene unit from Northern Tool.
Someone, Land Pride I think, has the only unit I know of that comes with down-pressured rear hydraulics and a carbide fishtail bit tip. TBN member Ultrarunner has this unit. If you're going to be augering into hardpan, etc. you will need this as a standard auger and tip without downpressure will not bite into extremely hard material. I have seen a dealer do a custom modification on a Woods unit (for a BX) to give it downpressure, but they did their own plans, etc. & was extra $.
Most typical units will run around $500-$600; the Land Pride with downpressure is about $1800.