Don't weight and bodyfat% have high correlation to lifespan? You can get a good enough scale for both for pretty cheap.

Fitbits and such also measure resting heart rate for cheap. As far as I've heard, that correlates well to lifespan as well.

You can control your weight and bodyfat reasonably well on a diet that consists solely of Twinkies, so only measuring those won't tell you much about the quality of your diet.