I believe that 2.3 Mustangs make great daily drivers. You get the look, feel and handling of a sports car, and the fuel economy and over all maintenance costs of a run of the mill 4 cylinder. As for fuel mileage goes, I think most of us on here get about 22 - 25 mpg in the city, and about 30 - 35 mpg on the highway. I drove both, so I usually end up somewhere around 25 - 29 mpg. If you want to go with a 5.0, I dont think the fuel mileage is that bad. However, you also have to take into account things like insurance costs, tune up costs, parts. Everything is twice as much because the motor is twice as big. If you were to buy a 91-93 2.3 Mustang you would be getting stock 105 hp. Add a cheap air induction to that, and it is no slug. Of course, you aern't going to impress anybody with your 1/4 mile times, but works great for a street car.
P.S. - If you are looking at buying a 2.3 Mustang, I think everybody in here would recommend one with a t-5 manual transmission. A4LD automatics are for the most part, from what I have heard, horrible pieces of garbage. They will get the job done, but a t-5 manual is strongly recommended. Hope this helps.