Well first off, the 520 is a great bike for that and would normally be well worth it, except that the late switch presents possible problems. Two concerns - mechanical and fit. Mechanically, you run the risk of problems with an unknown bike. If you really know/trust the seller and/or can see it has recently been serviced, you should be ok as far as reliability, but you don't have much time to fix any issues that crop up. If it is 10 yrs old and hasn't been used in the last few years I would probably pass - too many things that could go wrong (dry bearings, etc.). If it is only a few years old, it should be fine. In either case, get it down to your local LBS for a service if needed, and take a good ride after that to make sure they didn't forget to tighten something.

On comfort, the 520 is very comfortable for most folks and assuming it fits you, there shouldn't be a problem. I would think it would be more comfortable than an old mtn bike, especially if you hit headwinds. If it is a fairly new or a well kept bike I would probably say go for it and have a great ride.