That's not a bad price on the 500 if it runs as claimed.
I had a LT250R. It ran pretty good. I did not have any porting work, just the bolt on mods. I think if you come off a modern 450 to a LT250R it will be somewhat disappointing (I have a YFZ450) in the dunes. There are guys on here that can make the 250 into a monster that will have no problem hanging with a 450 but its going to cost some time and money.
The 500 is beast if it has the proper modifications and durability issues fixed. Again - time and money.
My bottom line is this: Inspect the Zilla, if it runs well, that not a bad deal. You might have to address some issues at some point. Understand what you'll get out of the 250. If you just want to dune and hit the trails asap. Find a clean 450. They are super cheap right now.
Im in Wayne Co. by the way!! Welcome!!