Price matters. I bought a BX2350 in an auction for $2K. It wasn't even clear that it ran, and every panel on it was broken (it was the model with plastic panels). It ran without missing a beat for 3 years, then I sold it for more than I paid. Kubotas are tough.
The seller may know more than you and not be sharing it. Or he may be genuinely telling you exactly what's wrong with it, and he had no issues with it so he did nothing about it. A machine with low HST pressure will run just fine, maybe he never got around to doing anything about it.
600 hours sounds low, but it is possible. It doesn't look like it's been in a rental fleet, you'd typically notice that. Even if it had 2,000 hours, those TLB machines are tough.
SSQA only matters if you're swapping the bucket for forks or whatever. Some do, some don't. And you can get a pin on quick attach if you're so inclined - limits you to implements you own and modify, but it works. I did that on my B2601, and modified my forks to match.
Bent tie rod. Sure. My BX had lots of bent things. Didn't stop it running. I wouldn't pay new prices for a beat up machine, but again that's a price discussion, not a "don't buy" problem. Of course, in my part of the world these machines are hard to come by, you don't really get to decide to wait for the next one, you never get one if you do that.