I notice the 3pt is well braced. However it wouldn't take long to fold that up if you back into a tree or something (i know your not supposed to do that).
A little flex built into it might be a good idea. I think I would give the flex coupling at the top link a try, rather than mess with the design of the mower.
As far as the price, I was going to say $200, but that is without seeing the stump jumper. If it didn't have one, I would pass, but most of my cutting is on very rough stuff.(clearing property). Turn it over and inspect that on any future purchases. Sitting as near the ground as they do, they tend to rust out, and also take a lot of abuse in the normal routine of mowing.
The picture of it on the trailer says a lot more. It means it is well worth the $250 that the seller thought it was worth, since that is what he got...
Get out the wire brush, a good heavy duty one, on a right angle grinder, and get to work. You can play with it later, but for now, stop the rust.
David from jax