I would try to climb on a naked old barn as little as possible. Think about scaffolding, or even a man lift.
But to answer the question, I like this one
Petzl Navaho . It's pretty compared to the cheap ones.
Also, a fall arrest system is worthless if you don't have someone with you to get you down when you do fall. And that is not nearly as easy as it may first appear as many people are knocked unconscious when they fall, because the shock absorbing lanyards lengthen considerably when deployed and they hit something below them. Even if you don't bash your head in on the way down and what you're tied to doesn't fall on your head when you suddenly and painfully decelerate (tie off points have to be capable of withstanding 5000#), the way you dangle in those things often can lead to asphyxiation within minutes. Very scary. Make sure you are properly trained to use the gear and have someone around who is trained to rescue you.
That's why I would be looking for another way -- secured ladders, scaffolding, man lifts, or even a proper work platform on a telescoping forklift are some safer ways to access the higher stuff.
Work safe and have fun -- I'm glad to hear you're going to make use of the old timbers.
Gabel