Thanks to all for suggestions on vendor, and on tips.
Especially appreciate experts (each with their own viewpoint no doubt) such as A Stewart, Retro Grouch, Thermionic Scott, cyccommute, Mad Honk and others. To Andrews point, I obviously have no expertise in wheelbuilding, but I do have a pretty good tech and bike (engineering degrees and bike shop experience) background, so when things are mentioned I think I grok them pretty well. So I appreciate your sharing your expertise.
Summarizing some stuff:
1) There are lots of recommended vendors, with
Yohimbo's Garage and
Dan's Comp being mentioned a lot.
2) Many mentioned that I should remove the rims from the hubs and check flatness and roundness.
3) It was pointed out that buying a whole wheel might be cheaper in most cases. (But I want this to be close to "stock")
Some more technical stuff.
1) The issue with butted vs straight gauge spokes is interesting. Andrew pointed out a key point: Butting is more for reliability than for weight. Reliability is from
a) Butted spokes will stretch more for the same force. This means butted spokes transmit less force to the the j bend, which is where most spokes break.
b) The amount of stretch from tension in butted spokes is more than for straight guage. So if the wheel is displaced, the butted spoke is more likely to remain under tension.
2) The same "amount of stretch" advantage (1b) accrues to longer spokes. Longer spokes have more stretch length than shorter spokes, given the same tension.
3) The 4 cross approach may be either spongier and less snappy, or more comfortable, depending upon point of view. But a lot of this difference may be irrelevant, as tires soak up a lot of force and dampen things a lot. Perhaps more than the difference between 3 and 4 cross.
4) Another advantage of 4 cross is that the spoke force on the hub rim holes is more tangential and the spokes oppose each other, and this makes it less susceptible to hub hole break.
I tried to think a bit about what others have said, and it seems to me that lateral, radial, and tangential stiffness is going to be proportional to the spoke cross-section, but that much of the road-shock absorbing flexibility is going to be less important than tire effects. The tangential stiffness needed when one stomps on the pedals probably does matter, though. So for comfort and reliability, a butted spoke is favored. For rapid response to graunching down on the pedals, a straight gauge (or less extremely) butted spoke becomes more desirable.
One interesting paradox. I'm a heavy rider. If I hit a bump, the rim will deflect more than it would for a lighter rider. This means I need more initial stretch in my spokes to absorb that deflection without going past the "zero spoke stress" point. So I (the heavy guy) need a butted spoke. But a lighter rider won't need as much pre-stretch and can get away with a straight-gauge spoke! This applies to the likelihood of breaking the spoke at the J-bend, too. The heavy guy is gonna want the thinner spoke section to avoid transmitting a lot of force to the j-bend. A lighter rider will be generating less force and can use a straight gauge! Well, I thought that was interesting.
The last consideration I had was for asymmetric lateral stability in the rear wheel. If you dish the wheel properly, the DS spokes are close to right angles to the wheel axis. The NDS angle is a bit less. A force displacing the rim towards the NDS will meet with less elastic resistance than will a force from NDS toward the DS. My understanding is that most of the force of the freewheel/freehub is transmitted to the rim via the DS spokes, as the thin section of an aluminum hub won't handle much torsion. Both points make an excellent case for asymmetric spoke selection for the rear hub, with thicker DS spokes.
I finally pulled the rear wheel and it has a Normandy freewheel, for which I don't have a tool. I'll be taking this to the bike shop to have them pull it, after which I can disassemble it and measure stuff. BTW, I saw a cool YouTube video in which a guy took a castellated nut for a 5/8" thread. The nut corners match up to the Normandy freewheel splines! You put the nut in, and use the slots in the castle (with a piece of flat steel) to twist the thing. Cool to know in an emergency.
In any case I think I'm going to try to match the OEM spokes on the front wheel. Butted, cuz I'm a big guy. 4 cross, to avoid hub hole pullout and for comfort (the wheels are very comfortable to ride right now). I'll use butted on the rear hub, but I'm going to use the different angles on the DS and NDS hub sides to figure out a good ratio of spoke cross sections. Will report back.