Hello and welcome to our family!
Always good to see people starting modding! But let me warn you, its more addictive then playing games. I have reached my 600h´s in Creation Kit just yesterday, so better bring a lot of time
Now to your idea: A build your own stronghold is a nice idea. Myself, I work on a Hearthfire styled fort with player home, in short, a build your own fort mod. Believe me when I tell you, its a lot more work then you think, hell, even I didn't believe how much work it is (but I love working on it). And transforming is even more work and also a good way to get bugs. So if you really want todo that, let me give you the following tips:
Write down what you want to do! On a piece of paper or a words document, doesn't matter! Just keep track of what you do and split things in tasks. With scripting and all the build able stuff, it can get quite over overwhelming. Which brings me to...
Name your items, objects, references, etc in CK in a certain way or system so you find stuff easy! I have about 25 build able objects so far, with a construction recipe for each one. And I only have the basic stuff so far! Also, I haven't even started the player home! So that will be at least 10 new for the outside plus I don't know how many for the inside! So keep track of what you make!
Save often and test it in game! Whenever I am done with a new building, script, etc I save it, clean out any new bugs I can find (use TES5Edit) and test it. If it works, I save it in a backup folder and then start working on the next part. If it doesn't work, I fix it and don't work on something else until then!
Think of the NevMeshes! Don't save them for the end or you will have a bad time "fixing" them.
These are probably the most important things to consider! At least in my eyes
Also sorry for my bad English, I haven't slept since at least 20h and yeah^^