High mileage on a sporty car like a Miata, 240, etc, is a little bit more worrisome than on a family hauler. They've probably been 143K well-enjoyed miles, if you get what I'm saying.
That being said, I wouldn't worry about 143K, that's not a whole hell of a lot. I work for a race prep shop building Miatas for a Performance Driving School, and some of our cars have more than that, and they've been track-only for at least half of their lives. Just run a compression check on the motor, and change the belts and fluids if/when you get it. Oh, and at that age, if it's not
leaking a little bit of oil from the rear main seal (coming out the bellhousing weephole) then you've got a problem.