I have an EP-Carry and love it. Answering your questions one by one:
- What's the right shaft length for a DS1? (Guessing 36" would work): Pretty sure mine is 30", I'd have to re-measure it. 36" is fine.
- I notice that there are more powerful motors (70# or 80#) but they require 24V. Is the extra power worth the extra weight?: No.. see more below
- are there other brands or types of (electric) motors out there I should be looking at? I don't think I'm ready to make the price jump up to a Torquedo... but are there other 'electric outboards' that might be more reasonably priced?: Define reasonable......
On the last question, I don't mean to be flippant and I'm not asking how much you can afford. I'm wanting to point out that any auxiliary power choice is a tradeoff of initial price vs total cost of ownership vs convenience/does it do what you need it to do? So, to be absurd, the cheapest aux. propulsion is a used canoe paddle. The most expensive is a brand-new 100hp gas engine. Both have their pros and cons (100hp engine is going to make your boat get up on a plane, then be totally uncontrollable and flip). So, what are you trying to do? Power the boat all day while transiting from X to Y straight upwind or on a windless day? Do you expect to be fighting against currents? Or do you just want to get out of the marina/away from the boatramp and head upwind while you raise the sails and then at the end of the day want to putter back to the dock after dropping the sails? I suspect most DS sailers want the latter ability and that's it.
If so, the EP Carry is a great option, check out my post on it here:
https://forum.daysailer.org/forum/viewtopic.php?f=8&t=5936. Yes, it's $1600 initial cost. On the other hand, that's ALL it will cost, except maybe for an $80 prop change a few years down the line. I also got an extra battery for $60 (by scouring the internet for refurbished wheelchair batteries, I got the exact same make as what EP Carry uses. And even got some tech tips on it from the company, they are very good on working with their customers). You really don't need a lot of power, a 70# or 80# trolling motor is way overkill. But before we get into that, answer the question, at least to yourself, of what are you REALLY looking for in aux power?