Personally I'd use an 1/8" aluminum composite panel for it (sold under several names....polymetal, dibond) The material has a solid plastic core with a thin aluminum skin on both sides. I like using polymetal instead of a thin aluminum because the plastic core keeps it from developing a wavy "oil can" appearance. If you can't round up a supplier for it use a regular aluminum sheet at least .063 thick.
I'd attach the sign panels to the frame with VHB tape so that it wouldn't have visible fasteners. Some people don't trust that and would use sheet metal screws or pop rivets. Whatever you want to use will be okay.
Vinyl-wise I am a firm believer in Oracal 651. The colors are good, it's easy to work with, and it holds up really well. I buy it from Fellers (
www.fellers.com). You can order it in 10 yard rolls. Make sure you ask them about free shipping.
Spend some serious time working on the design. You're going to have it out there for a while, spending a few extra hours making it look right is minimal compared to how long you're gonna have to live with it once you letter it. The vinyl itself is relatively cheap so don't be too worried about using more than one color. It takes a little longer but looks a lot better.
the only other advice I can offer is to fire Bobby Patrino.