is there any reason I can't use Monopotassium Phosphate and titrate with KOH up to the desired pH range?

I believe so, but if there is I don't know it. Anyhow, I found some dibasic potassium phosphate on Amazon: http://www.amazon.com/Potassium-Phosphate-Dibasic-Grade-1000g/dp/B00CF28K2G

For a 1500L Hot Tub how would I know how much to add without gross trial and error?

Dimensional analysis and the Henderson-Hasselbach equation. Or, use this (just be sure to change the molecular weights).

Most literature recommends a TA (total alkalinity) of 100ppm for carbonate/bicarbonate buffers. Would the "total alkalinity" of a phosphate buffer correspond to a carbonate/bicarbonate buffer?

I have no idea about this part.