new casters for some car dollies? Here's what I'd do in the same situation. Work out the bolt pattern for the new casters and prick punch the first hole. Center drill on dimple, zero dials/ DRO for both axes. Drill hole, move one axis, drill next and so on (easier with DRO, if not go back to first hole before changing axis). Swap out drill for countersink, repeat. Swap out countersink for tap, power tap. Done. If you have the travels then you can even do all of the holes on one dolly at a time. For the holes on a slope, find a centercutting endmill closest to the tap drill size you want and use that, possibly for all of the holes if it saves a tool change. That would take perhaps 30-45min per dolly, maybe less for the later ones as you memorise the numbers. Moneywise, that depends on what you'd want to charge in the first place. Based on my own "shop rate" (ha, charged once!) of $25/h, that'd be $80-100 for a set of 4 dollies.
As for the height thing, I feel for you. I'm 6ft2, so everything in my shop is set up accordingly - bench height, mill etc. For my mill I made a moveable base out of 2x2x3/16" steel tubing (I think) and 1/4" plate, with casters and feet to jack the casters off the ground when in position. Raises it up about 3-3.5" which helped a great deal.