You're short a 0. 500 l/h filter probably wouldn't even have enough head to pump any water at all into the tank. You'd want a minimum of about 5 times an hour turnover at the tank, which would be 3750.
Yup, get an RO. DI gets used up extremely quickly if it's soaking up 1000PPM from straight tap water rather than polishing off the last 5ppm after an RO filter.
I'd go with something like 1-2 Laguna PJ5000...Actually, I don't know if the still make them, but something in the 5000-6000 LPH range would work. I used one on my 300L marine, gave a good amount of current all around the tank.
I don't know if I'd call them a dime a dozen. Maybe your dimes are worth WAY more than mine. After some investigation it looks like they're about $400 NZ each and out of stock on the couple US sites I could find that advertise them. Given that fish prices for FW fish in the US are generally half what we pay here or less I'd be surprised at anything less than a $800NZ bill for one here. And that isn't out of line with the prices I've seen for them in the past.
Nope. Has either no effect or slightly softens and acidifies the water.
I put coral and bird grit in my filters to keep the PH from dropping too much with my rainwater. Otherwise it drops off the bottom of the scale.
Live sand and rock should be the last thing you get. Otherwise it's going to die sitting in a bucket for months waiting for you to finish setting up the tank.
Water won't effect the polyurethane. Plecos might, but they won't wear it off at a great enough rate to be a big problem. Just put a couple thick coats on it, it will be fine.
I've found, at least in what I look at, that a lot of the trademe "businesses" just buy from overseas websites, tack on a big profit margin and resell stuff here. Might as well just go straight to the source.