The "standard" (as in, most every sign blank sold as being for banjo frames and such) is .04" aluminum, and they work fine for the post/panel setups, too. And they pretty much always have a 1/4" or 1/2" radius corner (or they used to, anyway- haven't done a real estate sign in something like 12 years, thank goodness).
3mm ACM, if it actually works out that it's cheaper for you to cut it yourself (as opposed to buying the .04" blanks) would actually be an option you might be able to upsell, as the thicker material gives the customer the impression that they're getting something "beefy".
Personally, I'd avoid coroplast on that type of sign. It would just look all kinds of cheap. If they're using step-stakes, sure. Even banjo frames, but not on a hanging sign that's supposed to look like it's a quality product.
PVC would be fine- but only if you use the outdoor quality stuff (there's a specific name for it that's eluding me). The cheap stuff will almost certainly warp, especially if covered with a dark colored print or paint that's just soaking up the sun/heat.
MDO will only look nice until it warps. And it will.
All of that said, though, your real dilemma (at least in my experience) is that you'll be hard pressed to convince an agent/broker to pay for the quality you're wanting to provide. They are (again, in my experience) some of the stingiest, obnoxious, most P.I.T.A. customers you can have unless you've contracted with the agency itself to a rigid set of specs that the brokers/agents are expected to adhere to. You can come up with the best possible solution as far as looks and durability, and they'll still be trying to nickel and dime you on every single one of them.
If they do buy from you, they're also going to generally abuse the $#++ out of what you give them a fully expect you to replace them stuff for free- like when they toss it in the back of their minivan on top of their kid's bike and the pedal gouges a tear across the face, or the post gets cracked when they're backing up that minivan to remove the sign and roll over it- except what they were really doing is looking at their phone, or their picture "doesn't look right" (even though they're the ones- or their 'cousin who's a designer'- provided a "print-ready" files and it was fine when they got the six months previous), or the S hooks "fell out". The list just goes on and on.
Man, am I happy not to deal with those items any more.