Once you get your head around what's going on its pretty simple. The max amount of fuel your FSD can swallow in a single gulp and its "optimal mass" depend on the size and grade of the FSD, as modified by engineering. How much fuel a jump uses up is a formula based on both the distance and your ship&cargo total mass compared to your FSD's optimal mass. If the jump requires the FSD to suck in more fuel than it can swallow in one go (or more than you've got in your tank) then you can't do that trip in a single jump.
The formula for fuel cost in a jump is not linear with distance, at the same mass and with the same FSD a 20ly jump will use up more than twice the fuel that a 10ly jump does. This is why the route plotter offers you the choice of "fastest" (fewest jumps) or "most efficient" (least fuel) - an efficient plot will usually get you further but require more jumps to do it. This can, for example, get you through a zone full of unscoopable stars without running out of fuel midway or allow you to extricate a ship with a short jump range from an isolated star system that it can't make it out of on a fast route.
As for Horizons being "worth it" or not, if you never want to land on a planet (for example, to visit an engineer to tweak your modules) or don't want access to all possible ships and gear, you can live without it. However, all future developments in gameplay will be based off that foundation so unless you're happy with what you have in the game now being all you will get, you probably do want Horizons. You might be comfortable staying a generation behind in the name of reduced cost, so be prepared to wait out the next major development cycle in the hope that Horizons becomes cheaper in the same way the base game did once Horizons was established but that could be a long and frustrating wait.