Ive read the buyers guide but I want to be sure

I dont care about premade adventures. I've played through some and id be happy to buy a premade adventure here and there (Such as Curse of Strahd or TOA) but there are many I ahve no interest in

I want to have access EVERY class/race/spell/feat/item that is available in officially DND thus far outside of homebrew obviously.  Do I get this with just the sourcebooks or are there some class/race/spells/items that come from some of the premade adventure books and will I have to fork out for the lot?

There are some spells/backgrounds/items in the adventures too, but they are really a minority (there are 3 spells in the IwD for example, but I don't remember any more spells in the other adventures). As for the classes/races, they are only in the source books iirc

If an adventure introduces new spells, magic items, races, etc then D&D Beyond allows you to buy them piecemeal individually (or as a bundle with discount) rather than paying for the whole adventure. 

For example of Icewind Dale: Rime of the Frostmaiden, there are three spells. If you just want the spells you buy them individually for $1.99 each or as a bundle for $3.99. Anything you pay is deducted from the cost of the book if you do later change your mind and want to buy it, so there's no risk or paying twice.