I bought a used 955 Cat several years ago and spent 3 years changing the shape and contour of the dozen or so acres where my house is as well as digging a lake that is just over an acre in size. Sure I ended up doing some work to it; as expected, but it wasn't bad. I had both tracks off at different times, rebuilt the lift cylinders (one day and under $100) and replaced the front ilders. That sounds like a lot, but I pretty well ran that thing 3 or 4 hours a day on average. It worked a few hours every evening and all day on weekends. When I was completely finished I sold it for a profit, including all parts I bought for it and even fuel and grease costs.
If you know how to determine the condition of an undercarriage and if the transmission feels right, if the hydraulics feel right and slop is minimal, you can certainly buy a used piece of equipment that isn't ready for the scrap yard. I'm actually considering the purchase of another dozer or excavator for a couple hundred acres I've since bought. The problem is that I now need something that will be far too big to easily transport. As a matter of fact, I'd have to get a permit every time I move one the size I'm considering. That kind of kills a bit of the idea. Otherwise, if you know equipment well and can work on it if need be (or know someone who does and can) I would still consider the purchase of used equipment if you think you'll need more than one specific thing done. For me, renting one for several months would have been out of the question and there was no way I could have come up with the money to have hired out all the work I did.