I have not purchased a mod yet but I will, so if I do, will it effect all the worlds I've created? Because I may not want Daleks (from the Doctor Who mod) running around in my survival.
Do mods on Minecraft effect only one world or all of your worlds?
First of all, just about every mod out there's FREE. You don't need to purchase it! If a site is trying to sell you a mod for Minecraft, you're probably getting ripped off.
When a mod is installed, it will affect every world that is loaded while the mod is active. For example, I have two worlds called "taco" and "truck". If I load "taco" with the Taco Truck mod (doesn't actually exist), then I should play "taco" with the Taco Truck mod from now on to avoid world corruption.
If I uninstall the Taco Truck mod and load "truck", it will be exactly as it was when I last left it.
For starters, don't purchase mods. They should all be free, and any mod that isn't is trying to scam you.
If you load a world with a mod loaded, that mod will be written into that world. When the Daleks mod is active, don't load your Survival world and it won't be effected.
Make sure to re-enable the mod before you try to load the world that does use it though!