Hi,
the UB300 is perfectly good, although it misses on a few features that the ESS has, but is none the less perfectly OK.

CNCRoom may be out of stock of the ESS, given that the ESS is US made, but surely they have their own MB3 board? If that's the case you can pick up an ESS anywhere, including direct from Warp9TD.

Just to be sure, if I get the ESS/MB3, or UC300ETH-UB1, that becomes my bob and I just attach my existing wires to it and the C10 goes away. Connects to my laptop and spindle runs on P1/P2 of the KBB. True?
Yes. I'm assuming the UB1 has the same Anaspeed circuit...seems probable.

And if I were to make the circuit you so generously shared, I can use it with my C10 and get the spindle going, but I would need to add a board like the UC400ETH to attach to my laptop instead of the old PC. Yes?
Yes, that is correct, you'd retain the C10, and add the little extender board but you'd still need a motion control, either a parallel port or one of the ethernet or USB connected motion controllers.
I'd avoid the cheap Chinese made motion boards, they are more hassle than they are worth.

If you are inclined at some future date to upgrade to Mach4 then the ESS has features that the UC300, UC400 and UC100 do not, to the extent the the price premium for the ESS is moot.

Craig