Although I am not an expert mechanic, I have learned one thing in the 50 years or so that I have been messing around with cars and that is when approaching a project like this it's going to cost more than you anticipated. Get use to the idea. Don't go cheap, as Eric has said when describing the failure of a used valve, it's just not worth the downside to go cheap because it will almost always bite you in the a.. when you aren't looking. Even if it takes longer than you thought, in the end the piece of mind will be worth any extra expense. That said, of course a new part can fail too, but the likelyhood of part failure is greatly reduced. Good luck!