Rich, I'll start off with I like the E310 and I'm presuming the internal parts, Burners, Flav Bars, Grates are in decent condition.
Fixing the rusted section looks like a little bit of work, but worth it.
here is what I would do, given your comments of the rainy and wet weather.
I would dismantle the grill down to the frame. Remove the doors, lid, grates, Flav Bars, bottom pan, grease catcher, basically anything that comes off easily.
I'd pull the side tables, and remove the manifold and the burners. Don't remove the screws in the cookbox that the burners slide into. Unbolt and remove the cook box.
What's left is the metal frame and then you can attend to the rusty sections. easily.
I can't tell how bad the rust is on the front legs, but the back legs may also be rusty. If the bottom of the leg is rusted too badly to hold a caster, I'd suggest you post close up pics and you'll get a few suggestions on how to splice in a new section.
The advantage of dismantling it completely is it will be much easier to work on with the weight of the cookbox, lid and internals removed.