The 23 series retained a lot of Hobart influence but got an upgrade with Whirlpool's power module, so it's considered better than the 21 or 22 series for that reason.  It should be a fine placeholder until you find a true Hobart, but I would steer clear of the 21 or 22 since they (and the 23) offer only a single final rinse.

Your ideal Superba would be an 18 - 20 series, but be advised hat the 19 series has no heated drying option if that makes any difference to you.

Try running the machine through its longest wash cycle with some Lemishine powder and see if that helps. 

I was able to find a small jar of Maytag (no relation) blue plastisol touch-up for an issue with rust on racks, but that was several years ago and I located it at an appliance parts store with a lot of old stock.  The container looks similar to "White Out" and includes an applicator brush.  It may take some searching on line to find it.

Otherwise, racks from older models (back to series 20, if not futher) will work if you can find a donor machine.  I'll leave the specifics about model range to the experts.

You definitely know what you're doing and I'm sure that once you get your hands on a series 20 Superba you'll have it operating like new!

I think the dull interior is a fairly common thing among vintage dishwashers, and that it's usually an indicator of how much it was used or water hardness. 

While the 23 series was the first to feature Whirlpool's improved Power Clean module, I'm pretty sure it still provides only a single rinse.  The 20 series was the last to provide an additional rinse.