I totally agree the nutes will be costly. My reservations are:
-Do the HALOs (aka drip splitter things) have maintenance, ie clog.
Actually they don't require much maintenance, The splitter thing to which I was referring is this
These are the ones I actually use and they work great, very easy to disassemble and clean if needed but I've only done that once and only to check, wasn't really clogged. There's one with 8 outlets but I couldn't find a picture.
The halos are used to deliver the water to the plant, not to turn the single outlet from the pump into whatever number of outlets you need for your plants.
Anyway, neither of these elements clog because there's nothing to clog them, maybe if you don't clean the entire system in a couple of years they would, I don't even check mine anymore, I'm that confident and haven't had any problems and water flows just as good as it did on day 1.
-How often do pumps fail? And what size pump? I don't mind paying more as long as it's reliable.
Pumps don't really fail as long as they don't suffer from a severe power surge that may damage the coils or water gets in where it shouldn't (which doesn't happen unless you physically damage the seals which are rock hard resin.)
The size will depend on the size hose you plan on using, for my veg I use a 623 gl/h pump which is way more than what you'd normally need, but the way I have it set up I have to pump the water up two levels, or shelves if you will, so I need a little extra power. I use 3/4 pvc pipe to deliver the water to the distributors and then 5mm polytubing from the dist. to the plants.
This pump is a Sicce 2400 (or something like that), it a very good brand and not really that expensive (I think they go for about $50 here) I've had it for 5 years and I used to use it for my aquarium. I never had a water pump go bad so they're pretty reliable.
I tried with a smaller pump once and I learned that's it's much better to have a powerful pump and regulate flow through valves.
-does this require a great deal of 'tuning' or 'trimming'. With ebb and flow everyone gets dead on equal watering. I can see a lot of variation occurring
It does require some initial tuning if the pressure is not enough. With a 600gl/h however you should not have any problems at all, anything over this will provide way more than enough pressure. If you fell like spending a bit more, go for a 1000gl/h to be sure you don't have pressure issues.
I actually have a valve set up just before the distributors, they are open only to about 25%. If I open them any more then they pump too much water for me. I say to me, because since I only use the cubes, if I let water cover the top surface they algae grows in there, so I have to make sure that water gets delivered from beneath. I accomplished this by attaching a drip accessory to the polytubing which allows me to bury the outlet a bit so water doesn't flow over the surface, exposing it to light, thus creating algae. If I open the valve to full, water would flow everywhere and make a mess.
For a system like yours and if you use halo I would probably stay at 100%
Variations between plants should be minimal and I'd label them as negligible.
-does it/ is it necessary to run it 24/7? can it it be run 30 on/30 off?
Run it 3 times a day, 15 min at a time or you'll over water them and have serious issues later.
Have you looked into GLR? it's gonna save you a lot of power and the logic is sound.
-why does it have to sit off the surface? Would a ring that sits on the surface be as effective?
I assume you mean the halo? I don't suppose there's a difference between running it from above or onto the surface, but what I'd probably do is bury it under a layer of clay, the water should have a minimum of light exposure to avoid algae, so burying it would be a good idea I believe.
-my pots are 8ish inches across, why are the smallest halos 6inches? It seems unless my roots are already established and deep this would completely miss the plant all together? (large concern here)
Water is going to reach everywhere, the clay you have on top is going to distribute the water all across the pot, you shouldn't worry about this. The only concern I'd have is your ability to remove the halo once the plants get bigger? will you be able to lift it and remove it with the plant in the way?
You're gonna do great.