Compared to the 4016 you had asked about this seems like a much better machine. 4020PS with an under-the-seat PTO is pretty much the peak of the Onan era tractors. Especially with the gusseted rear axle! I'm not sure if that was standard on the very last years of the Onan tractors before they went black frame or if that was a warranty replacement, either way that's a nice upgrade.
The steering bias L-R is adjustable with the threads on the cylinder as you mentioned. PS machines tend to wear out the front axle pivot more than non-PS machines due to the potential for extra steering load from the cylinder. A new pivot pin might help tighten it up.
Hood not latching can be a couple things, the first thing I'd look at is the bottom of the hood at the hinge, overextending it (past the hold-open stops) will bend this section of the hood and mess up the geometry. Fixable but a bit of a pain. I wouldn't let it push me away from a sale but I'd definitely talk the guy down about it.
Seems like a solid buy, especially with the included attachments. Should be able to get some of your money back out of it if you don't need all of them.