Not really that much to maintain. More than a 152 sure, but that is obvious. Nothing like trying to maintain a complex. The 300 is fuel injected, the 260 is not. About the same fuel burn since you'll be able to more accurately control the 300, and with the extra power, it'll get you maybe 3-5kts more depending on who you talk to. I know guys who plan 130kts with a 260, for a clean well rigged plane.
I'm looking for one myself actually. I think it is the best value out there if you need something that'll hold full fuel, and fill up with 4 people and their baggage. And need to go up to higher DA fields (I have a place at 4500' I'll be going a lot) every now and then. Nice ones built in the 60s, mid time on the engine, run from $60-80k. From that point, you are buying newer, or better panels, paint, interior, etc.
One thing is check the fuel selector. All those 4 tank types in Pipers can develop a leak at a seal, and if they haven't been replaced, likely will need it. That is something that'll be handled already with any well maintained plane though.
On
www.myairplane.com you can do a background search on the tail number you are looking at. It'll give you the basics, possibly any damage history.
www.aso.com
www.controller.com
trade a plane, and whatever other sources you have.
Good enough plane for Piper to bring it back into production. If I had the money to pick a new, reliable, piston single, the 6x is what I'd pick at this point. Don't have the money, so I'm looking at a used Cherokee 6.