100% agree with staying away from the 6.4 motors!! I ran the 6.)'s for a long time and had very few issues with it but the 6.4's were the biggest waste of money I have ever spent!
If you can get a good deal on the 6.7 Power Strokes you will be happy for a long time. I have had several of those for years and only maintenance had to be done to them and they are getting close to 200K on the clock now. It is worth the extra money if you can do it because they will "love you long time, soldier!"....LOL