I like the water tower Idea, good use of space and gravity saves using one pump for water transfer to the mixing barrel.
the hoist kill switch sounds good. but I might have used a sand bag counter weight to balance the hood weight to neutral, instead of buying a power hoist. kinda like a sash window. then have a tie down at the up and down position to keep it in place. actually, make the couter weight just a little light and tie off in the up position. I like to keep costs down. But power hoists are cool!!
mounting the ballasts outside is a good idea, they last longer if kept cooler, and away from the humidity under the hood. Do those sump pumps have to draw from the bottom of the rubbermaid? I suppose so. well, back to worst case scenario of a spill. you could raise the sump and pumps off the floor a couple inches on bricks to prevent a possible short and hot foot when you hit the wet floor.
speaking of humidity, I always paint my hoods, inside and out with first an oil based primer, then a couple coats of oil based rustoleum glossy white. I see you are not using the individual T-5 bulb parabolic reflectors, I know, they are expensive. but if you paint the hood glossy white, you will protect the wood from discoloring and warping or delaminating from the humidity at night, AND you will get more reflective light into the tank. plain wood doesn't reflect much and remember, half of the wattage of light is going UP from the bulbs. glossy while oil based paint is a cheap, but pretty darn good reflector.
when you retro some halides later, they will make more heat, so you could drill or cut some 3-4 inch holes above them to let the heat rise out of the hood, so as not to over heat the paint or the tank. Natural convection, IE no fans to buy, run or replace. I like simple,when ever possible. If you mount the hood so that there is a 1 inch air gap between the tank and the hood, you will get better convection air flow to remove heat. a tight fit will keep it in.
Hey , speaking of retro fit HQI MH kits, have you seen the new ones on HELLOLIGHTS? they have a new HQI reflector that is just like a pendant, with UV glass, sockets, electronic ballast and power cord. these are perfect for retro.
http://www.hellolights.com/15hlmhrekith.html
for that future upgrade.
Hope this helps, you are doing a great job. And I hope my back seat engineer don't bother you too much. I like this post so we can all watch your progress. its going to be a nice set up.