Since you've gotten no answers, I'll try.
The HP mechanical standards are very, very good if used correctly and treated well. (few are treated so well in labs I've worked in). The specs for 85032B kit, is about twice as good as the one you reference though the 600 series specs are pretty darn good. It's like everything in engineering, what is "truly good performance". Return losses on a load over 40 db are very good, phase error of .8 deg are very good.
If you torque these to spec, keep them in the jewelry box out of the way, they'll probably be great. In my experience, 3.5 mm stuff has the shortest life, APC-7's are better if careful and N connectors are pretty bullet proof.
I think this new sets of Sigilent's would likely be better than something you'd find used anywhere. When you get them, do some repeatability tests and send them back if they don't repeat.
The pricing is pretty enticing. I have accumulated several cal standard sets over the years in all formats. HP, Maury and Gen Rad. I have a few lab made ones that are adequate for most of what I do and they can knock around the lab. (<2GHz). These standards at their price might be a good starter set and could turn into lab standards if you upgrade to something fancier later.
The specs on these are close to an 8753's base specs. The othter question is longevity- be kind to them.