Looks like a nice header. To start off you should probably just have a shop weld in the flex pipe. They are going to remove a piece of the header the size of the flex pipe before the o2 bung and weld the flex pipe right in. You don't necessary have to have a flex pipe but I would recommend it. If you are going to have a full custom made exhaust then you dont necessarily need flanges for the catalytic convertor so that would save you some time and money right there. As far as the resonator thats up to you. Resonators can absorb some of the sound to give you are quieter exhaust note and they can also change the tone and give you, in my opinion, a nicer exhaust tone especially with straight through mufflers. In most cases they place little to no restriction in the exhaust system. Anyways good luck with your exhaust, looks like you are headed in the right direction!