2-2.5" is fine. And actually you don't need more than about 1.5" in the front. You can slope it towards the back and you should have atleast 2-2.5" there, if not 3.
I'd go for it. If they offer free shipping now, thats a heck of a deal.
You will need the extra potassium because you won't be dosing much nitrates, especially once the tank is fully stocked. And if you don't dose alot of nitrates, then you won't be getting enough potassium. With the K2SO4, you will only be using it for potassium, so you'll be able to dose enough.
I doubt you'll find dry ferts at your LFS. If you can, then that is a fabulous LFS.
You may find the Seachem Flourish line there though, which are liquids. For a 75g those would be really expensive with the amounts you'd have to use. You'd go through the bottles really quick. The dry ferts, if you buy about a pound of each, should last atleast 6 months......and probably more than that. I use my dry ferts on 3 tanks and they probably last 6 months a pound, so I know yours will last longer. Plus, the dry ferts are pretty cheap.
I suggest you get a set of measuring spoons that have 1/4 teaspoon, 1/2 teaspoon, and 3/4 teaspoon atleast. I have a set that I bought at Walmart for like $4-5 that goes from 1/8tsp to a tablespoon.
A good dosing regimen would be dosing your macros (potassium, nitrate) about twice a week and dosing micros (Flourish or CSM+B) twice a week on different days (micros one day, macros another).
People that inject carbon dioxide, usually dose every day, but you probably shouldn't do that. You don't want to add too many nutrients without CO2, since that could lead to algae issues.
I suggest dosing about 1/4-1/2tsp of KNO3 and about 1/2tsp of K2SO4 on the macro days. You can check your nitrates shortly after dosing that amount and see what they get to. Before you get fish, or when you have just a few fish, you'll probably need to dose more nitrates, but later on the fish should produce enough waste to raise the nitrates.
Hope that helps...I know you didn't ask for a dosing plan, but I thought I'd mention that anyways.