There is no problem in hooking an RO to a hose bib, a quick trip to the hardware store should get you the parts you need. Pressure should be the same as you have in the house. Just keep in mind the smaller ones are less than 100 gallons per day, so having a reservoir would be an important item.
Also as far as buying a used one, you will probably have to replace the filters and membrane, probably cost you about $50. You might be better off with a new on off ebay.