First off, I think you should jump on it. 2nd, I wouldn't believe anything that dealer says about the car's history. I doubt the 150 miles are from customers. The bigger problem is from moron dealers driving it. I think the bigger concern would be the clutch over the engine, from moron salesmen driving it around. I estimate that any of us could take a Mirage and run it at full throttle for a full tank of gas and NOT do anything seriously detrimental to the engine. That is assuming the oil is good and full and the cooling system is in good operational order.
Nowadays, computers do a good job of stopping an engine from being abused. Now my old Scamp in the wrong hands, will fly apart. But that's old school.
As for the clutch, it's is easy to check, but I doubt it will have been abused even by the neanderthals employed by dealerships. Now, don't get me wrong, I don't think all salesmen and saleswomen are morons and neanderthals. Only most of them. When you test drive it, get it up to about 45 mph in 5th, and then plant the throttle to the floor for as long as you can. Like going down an interstate on ramp. If the clutch doesn't slip, you're good to go. I doubt you'll have a single issue with that car.
I too think most break-in procedures are overkill. I bought mine and immediately put it out on the interstate for a few hundred miles. And mine runs like a champ, very strong, and I'm at 25k miles. If I were you, I wouldn't worry about backroads. Slap it out on the interstate, hit the cruise and let it be rollin' and enjoy the cruise.
The one thing I was concerned with and pleasantly surprised about. I acquired a 1990 Celica ST through the perils of matrimony. It was a 5 speed. It would turn 4,000 rpm at about 75 mph. It was a buzzy b@st@rd and annoying at 4,000 rpm. But the Mirage engine is oh so pleasant sounding at 4,000 rpm. The road noise is excessive, but acceptable for the cost to join the club.
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View my fuel log 2020 Mirage ES 1.2 manual: 42.4 mpg (US) ... 18.0 km/L ... 5.5 L/100 km ... 50.9 mpg (Imp)