My company car is a 2006 Ford Focus Wagon. 180,000 miles and still feels tight and drives great. The Focus is a great choice for a DD. I'm thinking my wagon will be good for at least 200,000 miles and probably will make it to 250,000. The weak link on Focus' is the automatic transmission. Your's with a stick eliminates that.
My Focus still has the original alternator, water pump and fuel pump. The only things done to it so far are brakes at 85,000 miles, tires, new spark plugs at 125,000 miles, a new fuel filter every 50k, 2 batteries, a serpentine belt, the funky electronic air filter was replaced at 30k and has worked fine ever since. Oh a couple fuel filters, and a half dozen radio antenna's. They break real easy in car washes.
I travel a good bit, plus have a whole fleet of vehicles and I'm the only driver.
Here are some of the main issues you will face. First you start to feel guilty if you are driving the Mustang for mundane tasks. Don't do that. Treat yourself on occaisons and take the Mustang to the grocery store just because. Maybe one or two days a month take it to work.
My cars will sit for 3 weeks sometimes without being driven or started. Things are just fine with that.
Never start the car and run it for a few minutes. If it's sitting and you start it, get it up to normal operating temperature, the oil and water, which takes at least 20 minutes. If you run it a short time and don't get it hot there will be acid build up in the oil from the moisture of condensation and that will cause problems.
I would worry more about the gears not being in lubrication from sitting than the engine. Make sure everytime you start it you drive it a little bit.
A DD is just about always a smart choice and investment.