can be different for every project I’m sure, but planning things out is always the best method! Even if things change planning is always key! There are reasons to have a design document. 
Like yourself I am a modeler as well and have found BPs to make it easy for me to build simple prototypes and work in gameplay much more readily than I could with UDK. is the awesomeness and ease of use of BPs. There’s still a lot of programming concepts that carry over with it, especially when you get more involved with some setups, but it’s great for those starting out with game engines for the first time!
I’m not sure of any full on tutorials out there that go over an entire setup for a game from beginning to end with UE4, but some of things you want to do shouldn’t be that complicated to setup. You’ll need to setup multiple levels and menus though and it will require using BPs that cast information in order to track some things.
Let’s look at two of our example content provided on the marketplace that provides menus and start screens.
Tappy Chicken and ShooterGame
ShooterGame has a loading screen that is simply another map with a large plane mesh in it’s own map.
Tappy Chicken uses static meshes that when clicked by getting mouse position will perform different actions. There could be a menu with Tappy Chicken if that was setup in a different map and then when clicking start it loads another level map. The same for if you had multiple levels like the Angry Birds setup where you see Level 1-12 and clicking each number button will just load a different map.
For tracking information there may be something in the BP section of the forums or in the tutorials section that may help.