So, you're going to design for free - on spec - with the the type of potential client that is almost always the kind that cheap out... to potentially sell a sign?
On the legalities.. technically, you need permission to duplicate any trademark or copyright.
To me, it's just a bad idea to give that part of the work away to a person who has not even called you... any idiot can print something, you giving the design goods away is offering for free the thing that makes you different from the competition...
If you lack a portfolio (and work) make a few templates with made up logos and tell the potential client, your logo and information can go here...
In my experiences, real estate agents are the cheapest of the cheap, so they will love it when they get designs for free. That way when they call any other sign company, the first thing they will say "Oh, you know that sign shop Clem's sign-a-rama, that is doing our design for free, well can you make a sign like that, but beat his price?"
To the op: just expect them to call you up, make a change, and will want to see it, and then they will remember, oh, can we add the agents direct website at what ever url, and then oh, crap, I gave you the wrong url, can you change it, and on and on it will go.
I have always hated dealing with R/E agents, but grit my teeth, and deal. I am currently trying to finish a project putting vinyl on (3) electric For-Rent buggies that run around a popular vacation spot close to me all sponsored by real estate agents. The info was sent to me by a head agent, and I cut and paste all the info in, and in doing so, notice that she put the same e-mail address for (2) of the agents. Ok, I just go to the website, and get the agents info from their site, no reason to call, but sure enough, that agent wants a different e-mail... Oh, and wait, I gave you the wrong phone number for the real estate office, we gotta change that... oh, wait, can we change this...oh, wait, can we change that...