The two MDO 4' x 8's is about your only choice, unless you want to use some other substrate that will cost far more and not be that much more durable, but your going to need a third post down the center or those things are going to boing all over the place. You'll need to go in the ground at least 3 or 4 feet. So, unless your sign is going to sit right on top of the ground....
you do the math.
As for the installation, if this is your first shot at the big stuff.... are you prepared insurance-wise and tool-wise ??
You have to pull permits to start digging around on someone else's property and the first thing they're gonna want to know is if you have insurance to cover you and any helpers you have along with you. If they are part-time, then they'll need their own insurance. If they're subs or part-time employees, you'll need workman's comp and all kinds of other things before you can stick a shovel in the ground.
Don't forget to call the 800-dig number and get all your utilities legally marked.
There's a lot more to it than just pricing up some vinyl, substrates and labor time.
:Oops: Might want to make sure your design will fly by the codes department in case there are any restrictions where you're putting this sign.
