As reed.w mentioned, 6500k and 10k is a good mix of light colors and a lot of people use that combination. For an idea of how much light you'll be dealing with, see:
If we assume in a 75 gallon tank, the maximum distance a plant will be from the fixtures is ~20", with 2xT5HO, you'll be dealing with mediumish-high light. Some people swear you can get by without dosing anything additional in the water column, and some people believe you'll have an algae meltdown without dosing anything. With high light tanks, you can get away with a lot less.
Plants tend to have "favorites" for where they acquire nutrients. Some plants, like crypts with their dense root systems, favor getting their nutrients from the substrate. Other plants tend to prefer the watercolumn. But with a few exceptions (floating plants, mosses etc.), most plants will take the nutrients wherever they can get them. The most important nutrients are macro nutrients (Nitrogen [Nitrate], Phosphorus [Phosphate], Potassium), and there's a lot of lesser nutrients called micronutrients. Some of these are naturally in your water, some come from fish food....I've had much more in the way of algae problems though created by insufficient CO2. Whenever I've had a problem, most of the experts out there immediatly tell me to check my CO2, and then check again.
I personally buy fertilizer in bulk in powder form, mix up solutions, and dose using those. There's different commercial ones you can try out too (Seachem Flourish line, Pfertz...) but these are more expensive. Root tabs may do the trick too, but I've never used them myself.
Like most things in the aquarium hobby unfortunatly, there's no single "right" way to do things...there's lots of opinions and people have success with different things. A lot of planted tanking tends to be wading through through all of the information and finding out what works for you. Hopefully that helps - I know all the sheer information out there and all the little details can be frustrating.