Purchasing Ford F350 Dually, Need Advice

Sounds like you found a good truck. I'd be VERY leery of using it for a daily driver with only a 7 mile distance. The engine will never get up to full operating temperature. Diesels do best and last when they are run at normal operating temperature. Long idle when cold is also to be avoided. Newer diesels have a "kick up" to 1K rpm when the OAT temp is cold to get the engine up to temp. Most engine manuals will tell you to get a "load" on the engine within 5 min. and they also state to let it idle when coming off high speed/temp to get the turbo bearing temps down before shutting down. The engine manual for the Cummins diesel in my last boat was very clear about it. The manual for my Ford a little less specific but still states the same.
Best of luck, take care of it and it'll be around for a long time.