I'm in agreement with others here that a pulley system is probably overkill. The main reason is that with 3 workouts/week you will probably be able to do a single pullup within 3 weeks, if you dont miss a single workout. So, why sink money into a system that you will outgrow so quickly? I enjoy dyi workout machines, but I'd probably make one that had more long term usage.
Your time is probably better spent actually doing the workout to get to a single pullup than by building a pulley system. I would do leg assisted pullups. it isn't quantitative like you said. But a workout of 3 sets of 10 pullups, with your feet leaving the floor/stool at the end of each pullup, and with the last set barely assisting (as little as you can while still finishing the set). Repeat 3 times/week, and you'll get there.
If I read the situation wrong/you have a medical condition/ you still want a pulley assisted system, my apologies. I recommend looking into steel wire pulleys (similar style you see on weight machines in gyms) since after all you are trying to lift weights. Your pulley doesnt need to be able to lift your weight, but 2x the weight you attach to the other side. So if you're assisting by say 10kg, your pulley should be able to handle 20kg. Again, since it won't be lifting your entire bodyweight (in that case you wouldn't be doing any working out, now would you) it doesnt need to be rated that high.
Something like that.unless you are trying to assist more than 15kg, and in which case just get more of those pulleys, and have multiple weights (one for each set of pulleys).
Good luck! Update us when you get to one pull up :)