I have done some research recently on portable drilling equipment. The sense I get is that there are two "weak links" in the whole process that need to be considered. The first is the swivel, the part that connects the drill stem (i.e., the drill pipe) to the drive unit. The swivel also servies as the input point for pumping in the drilling mud. It doesn't take much imagination to appreciate how heavy the drill stem can get as the bore hole lengthens. Depending on the size of the drill pipe (more on that below), the swivel can easily be required to support a great deal of weight. In other words, make sure the swivel in the unit you buy is properly sized and engineered, and be comfortable that its limits won't prevent you from accomplishing what you want to accomplish.
The other weak link appears to be how well the debris that the drill bit generates is removed from the bore hold. That in turn is a function of how robust the mud pump is and how large the flow capacity is on the swivel and drill pipe. A small swivel and a long length of small diameter drill pipe = very minimal debris removal capacity, no matter how big a mud pump you have (or are willing to buy). I am no physics expert, but I imagine it takes a great deal of energy and flow to get drilling mud to go down and back up a 100'-300' foot long bore hole with enough speed to carry all the debris with it.
Of course, as you increase the size of the drill pipe to allow the necessary mud flow, the weight of the drill stem grows substantially. That brings us full circle to the swivel issue.
BTW, I already have a hydraulic auger drive unit that I use to drill fence posts, so I have been seriously toying with the idea of building a water well drill rig that will allow me to use the auger drive unit and my tractor hydraulics. Until I find a swivel, however, that is strong/big enough to do the job, and that I can afford, the project is dead in the water (no pun intended).