There is not a way to get a permanent ip on our minecraft server. That may change in the future - I'm not sure. But all of your mods are connected to your account so you can use them whenever you want.
But if you're planning on building permanent structures (like a house) in a world, you can do that. Structures will be tied to whatever world you put them in.
If you want to build structures that will be permanent in your world, you can 1. make mods 2. use the schematics block (under the entities tab in the Blockly menu) to save parts of your world. Also, there's a record kept of worlds that you've worked on.
You can always go back to an world and revisit it. To do that, click the My stuff tab in the middle of the learntomod homepage then scroll down. You'll see a list of worlds you have downloaded and worked on. Then you can download them however many times you want. The most recent world you've downloaded is at the top of the list.