OK, here goes, with priorities assigned (1=high, 5=low)
Go with the standard coated wire, or Heavy Duty?
Unless you plan to do hard-core ocean racing, standard coated wire is fine. Replace the shroud anchor pins and bow tangs. (Priority 1)
Several Hobie "pros" have recommended the Harken low profile 6:1 system, using 57mm blocks 2604 and 2629. Any reason why the lower cost 40mm blocks 2640 and 2612 wouldn't work (just about) as well? Are there Garhauer, Ronstan or Lewmar fans out there?
57 mm blocks are far too large. The current stock system (Harken 194 / 344) is quite adequate. Harken no longer makes the 194, so finding one may be problematic. Go through your dealer, as rumor is that Hobie Cat has stock. The 344 is a 29 mm carbo. The 2612 is an acceptable replacement for the 194. (Priority 3) There are less expensive alternatives, but Harken is the best, period. And they're made in the USA.
The swivel cam and traveler car need to be replaced. Does it make sense to upgrade to a Harken or Ronstan set-up, and if so, do you have recommended part numbers?
Replace the traveler car with the Hobie stock system. (Priority 1 if the existing one doesn't roll smoothly). If you want to burn some more money, replace the existing track with the deep-profile Hobie track (Priority 5) and "new" stock traveler. Make sure you get the right traveler car! (old stock has small, stainless "pin" bearings; new has larger brass wheel bearings). The swivel cam is a simple bolt on replacement - either Harken 240 or the Ronstan equivalent (Priority 1 if the old cleat doesn't work well).
I'm struggling using the Hobie catalog to find the right parts to upgrade here. The swivel cams and cars both need replacement (the original track is OK). Same question: what parts do I need to order? Would it be Hobie part 1075? And can I, without too much hassle or cost, add the traveler adjustment system, so that the cars don't have to be adjusted manually?
The Tren-Tec "plastic knuckle" cars are the way to go. Harken 240 replaces the swivel cams. Use Harken 468 cleats with the 476 base for the traveler adjustment system cleats (Lance cleats are an acceptable, less expensive alternative). (Priority 2) Use Harken 233 micro blocks for the traveler adjustment system turning blocks.
The stock system is two Harken 224 micro blocks hung off a small shackle. That's all you need. (Priority 2)
Any recommendations for upgrading the downhaul? Is the Power Pak overkill, and if so, what would be a reasonable upgrade from the ancient existing setup?
The 6:1 downhaul is not necessary unless you plan on racing. (Priority 5) A Harken 225 micro block hung from a shackle to give a 3:1 purchase will work just fine. (Priority 3)
Trapeze Kit and Harnesses
Any recommendations? We're looking at the "deluxe adjustable trap rig" from the Hobie catalog, but are there better, or more cost effective, setups? What about harnesses? We will not be racing, or sailing in extreme conditions (famous last words)...
You do not need a "deluxe system" unless you have money burning a hole in your pocket. Use the stock system with the addition of J&H handles. (Priority 2)
Trapeze harnesses are a personal thing. I've used The Hobie Convertible harness in the past and was quite satisfied. You can get much more expensive options. (Priority 3).
The previous original owner abundantly coated most of the mast, as well as the thru-hull fittings, with a greyish sealant, that is now ugly and rubbery. The Hobie dealer recommended removing this with acetone and a putty knife, and then re-sealing with clear 3M 5200. Thoughts or suggestions?
First of all, if you ever want to take something apart, don't use 5200. It's a permanent
adhesive, not a sealant. As for your existing stuff, it's probably just old silicone sealant. Acetone is useless. The only way to get it off is to scrape and gentle wire brushing. Then re-seal with clear silicone. (Priority 4 unless you've got leaks, then Priority 1).
One thing you have not considered is that with new rigging, new low profile main and jib sheet systems, you'll be wanting to carry a lot more mast rake. An old jib is not shaped correctly for this (too long in the foot). You should seriously consider getting a new jib (one that's less than 10 years old).