Here's the app I've been using:
https://itunes.apple.com/us/app/road-trip-mpg-mileage-fuel/id298398207?mt=8
A couple of other bits: watch the water pump & cooling system, and maybe have a spare water pump on hand (if you find a deal).
Just 'cause I'm a data/info geek, I'll also use the app Dash Command (
https://itunes.apple.com/us/app/dashcommand-obd-ii-gauge-dashboards/id321293183?mt=8) to monitor O2 sensors, exact water temps, and other parameters from time to time while I'm driving. If something is going to fail, I want to know about it early! It's easy to spot trends (poor mpg, high coolant temps, etc) in a regularly driven vehicle.